This reverts commit bb509134. We need to investigate intermittent failures in DiaSymReader first.

The signature decoder out of the dev/metadata branch will not ship in v1.1
RTM of System.Reflection.Metadata. Breaking changes to the API are coming
and the API will be officially reviewed and only checked in to corefx master
after v1.1 has snapped.

This temporarily forks the relevant code from corefx dev/metadata in to the
Roslyn tree as internal types. It sets things up so that dotnet/corefx#3560
being merged to master will be enough to get Roslyn using -rc builds out of
corefx master instead of -alpha builds out of corefx dev/metadata.

Fixes #5267.

* This updates all references to nuget.exe to use the copy in the root.
* This deletes all NuGet.config and NuGet.exe files that are not in the root.
* This removes all references to NuGet.targets (which is obsolete and no longer exists)

The targets files were previously split into an open version in
Microsoft.CodeAnalysis.Toolset.Open and a closed version in build/. This
unifies the four targets into two new targets, VSL.Settings.targets and
VSL.Imports.targets, and moves all the targets from ..Toolset.Open into
the build/ folder.

Update versions of Microsoft.DiaSymReader and Microsoft.NetFX20 packages to release 1.0.5 and1.0.3, respectively

Upgrades System.Collections.Immutable to 1.1.34-rc and
System.Reflection.Metadata to 1.0.19-rc.

We used to have multiple mechanisms - constructing a co-class directly
from a registered GUID, wrapping the co-class in SymReader (for some extra
constant mocking), wrapping the co-class in a TempPdbReader (IDisposable),
and wrapping a TempPdbReader in a DebugInfoProvider (trivial wrapper).

This change eliminates TempPdbReader and DebugInfoProvider and gives
SymReader exclusive access to the raw co-class.

This will be useful when we switch to ISymUnmanagedReader3, which the
co-class does not presently implement (but which SymReader can).

1. Compile them against Profile7.
   a. Swap out a few APIs (e.g. using GetTypeInfo for reflection).
   b. Introduce an abstraction layer so that we don't have to instantiate
   DkmDataItem (caused a VerificationException).
   c. Include WindowsProxy.winmd as an EmbeddedResource directly, rather
   than through a resx file.
2. Swap the output locations of the ResultProvider projects: Portable goes
to the main output directory and NetFX20 goes to a subdirectory (NetFX20).
3. Include the Portable ResultProvider assemblies in the EE vsix, rather
than the NetFX20 ones.

Rebased from https://github.com/tmat/roslyn/tree/DiaSymReader

This reverts commit b37052d0.

The primary use of DefineConstants should be to append constant values,
not replace them.  Any time a replacement operation is used it prevents
us from establishing defines in our targets and pushing them down to all
affected projects.

This moves our settings / targets files into the Open directory.  These files are a referenced by virtually all of the projects in the Open directory and are a necessary part of the build process.  Making this change is a necessary step in removing the project scrubber.  (changeset 1396832)

Removes dependency on MDBG from PdbUtilities and cleans up SymUnmanagedReaderExtensions and CustomDebugInfoReader. (changeset 1393106)

ImmutableArrayExtensions should be internal - we shouldn't be exposing public extension methods on collection types. (changeset 1392421)

The IDE uses four public APIs from the compiler layer for doing running analyzers in the IDE:

1. A bunch of static methods on AnalyzerDriver called Execute*Actions which take a type called AnalyzerActions which should be an implementation detail. So first change is to create a shared project called AnalyzerDriver that is referenced by CodeAnalysis and Features layers. I've split out these statics into a type called AnalyzerDriverHelper and made that internal. One challenge in doing this is that these methods create instances of Contexts and currently their constructors are internal. I've added constructors where possible and for some contexts that were taking Scopes, I've changed them to be abstract base classes and eliminated the scopes altogether. As part of pulling these methods, I needed some collections like ArrayBuilder, PooledDictionary etc and created another shared project called SharedCollections and put them there.

2. SemanticModel.GetDeclarationsInSpan - this is used by the IDE to calculate the declarations on which to run analyzers. This has also been pulled into the shared project and the API on semantic model made internal. We need a mechanism for branching into C#\VB implementations on both sides. Hence, the compiler API on semanticmodel still exists and is internal. On the features side, I have a language service, that branches into C#\VB and the implementation simply calls into the shared code.

3,4. AnalyzerDriver.GetEffectiveDiagnsotics and AnalyzerDriver.IsAnalyzerSuppressed() are two APIs about filtering that the IDE uses. There are legitimate scenarios for both of them being public APIs even if we make the IDE simply ask the compiler to run its analyzers. For eg: the IDE will need to run Document analyzers and the compiler can never run them. It needs to filter the results after that. Filtering requires pragma state and suppressmessage etc which are compiler'y things. The plan here is to move these APIs to a type called CompilationWithAnalyzers that John is creating to replace the AnalyzerDriver.GetAnalyzerdiagnostics() method. Once that happens, we can make AnalyzerDriver fully internal. This change doesn't touch those methods.

The shared code needed to access some resources. So I've created a type called AnalyzerDriverResources in both the Codeanalysis and features projects and they redirect to the appropriate resources.
 (changeset 1390439)

This changeset introduces the concept of a "global" nuget package. The references to the NuGet packages are inserted into the Open VSL.Settings.targets file, which is imported into every project in Roslyn. This ensures that every project automatically gets these "global" packages. (changeset 1386548)

Updates System.Collections.Immutable and System.Metadata.Reader to versions 1.1.33-beta and 1.0.18-beta, respectively.
     Updates VSMEF to the latest build from VSPRO_1.

     A few code fixes:
     - ImmutableArrayInterop is gone
       - in product we used in a couple of places where we now just create a copy of the byte[] (MetadataWriter)
       - test infrastructure uses a workaround with explicitly laid out union to avoid copies of large metadata blobs
     - EditorTestApp class was incorrectly importing MEF components resulting in a cycle. The new VSMEF reports such errors. (changeset 1385613)

***NO_CI***
 (changeset 1384854)

We had a number of projects that included the StyleCop NuGet package only to also disable StyleCop. We also had some projects that were referencing assemblies out of packages but didn't say so in packages.config, or had stuff in packages.config not reflected in the actual project file. (changeset 1378738)