by removing `<DebugType>` lines from projects that import `VSL.Settings.targets`

* 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* This removes all references to NuGet.targets (which is obsolete and no longer exists)

This moves all of the InternalsVisibleTo which were defined in code into
the project files where the majority of the IVT definitions lived.

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

This is the second of a sequence of changes.  (The first was 3dd1f2bf.)  In
this change we start returning custom type info (effectively just the
flags of DynamicAttribute packed into a byte array) from calls to the
expression compiler so that they can be consumed by the result provider.

TODO: The API for locals is still in flux and we don't presently have a
place to return the custom type info.

TODO: This change does not associate custom type info with locals declared
in the Immediate window.

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

Since hoisted locals are fields, they are in scope throughout the entire
method body.  However, from the user's perspective, they are still locals,
so we attempt to retain their original scopes during debugging.
Unfortunately, C# and VB use different mechanisms for doing so.  In order
to make the code more consistent, and to simplify consumption of the new
PDB format (which is more similar to C#), we introduce an abstract type
InScopeHoistedLocals that represents the set of in-scope hoisted locals in
a consistent way in both languages (and both PDB formats).

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.

In the dev12 PDB format, usings and imports were encoded as strings in
PDB files.  The strings were structured, but required a certain amount of
guesswork (for example, you might not know whether you were importing a
type or a namespace until after binding).  In the new PDB format, usings
and imports are represented as structured blobs and tokens are used
wherever possible.

This change introduces two new class hierarchies, ImportRecord and
ExternAliasRecord, that the expression compiler can pass around instead
of raw strings.  Unfortunately, there is still switching logic in two
places - both when these types are instantiated and when they are
consumed.  The instantiation switch is very simple - we just check for
the appropriate version of ISymUnmanagedReaderX.  The consumption logic
is more complex, because the new and old representations differ
substantially.

TODO: Presently, there is very little code for consuming the new PDB
format, since the APIs are not yet available.

In the dev12 PDB format, usings and imports were encoded as strings in
PDB files.  The strings were structured, but required a certain amount of
guesswork (for example, you might not know whether you were importing a
type or a namespace until after binding).  In the new PDB format, usings
and imports are represented as structured blobs and tokens are used
wherever possible.

This change introduces two new class hierarchies, ImportRecord and
ExternAliasRecord, that the expression compiler can pass around instead
of raw strings.  Unfortunately, there is still switching logic in two
places - both when these types are instantiated and when they are
consumed.  The instantiation switch is very simple - we just check for
the appropriate version of ISymUnmanagedReaderX.  The consumption logic
is more complex, because the new and old representations differ
substantially.

TODO: Presently, there is very little code for consuming the new PDB
format, since the APIs are not yet available.