We leave the assembly version at 2.0.0 since that's what we do for patch
releases.

We've shipped -rc4 packages to nuget.org, so any future builds should
be versioned higher.

We've previously stamped binaries with numbers like ymmdd, but for 2017
that would mean our number would be 70101 which is too big for some
PE file headers. We'll update to continue incrementing the month field
instead.

Fixes dotnet/roslyn#12038.

This reverts commit 436dd2c0.

We want master to remain version 2.0

Putting the GIT SHA into unofficial binaries was hurting our efforts around determinism.  It meant that binaries authored by developers which had the same code, but against different HEAD values, would end up with distinct binaries.  Hence anything in our infrastructure which leveraged determinism for caching wasn't getting the benefits it should.

The primary use of the GIT SHA is to help us track back from a released binary to the source that created it.  Especially important given the frequency with which we produce NuGet packages.  Hence it only needs to be present on official built binaries as they are the only ones we release.  Having it into the developer builds doesn't have any notable value.

This follows standard MSBuild conventions for file extensions:

- props: imported at the start of projects.  Primarily for setting properties.
- targets: imported at the end of projects.

This changes dotnet/roslyn to use a single NuGet feed.  Packages will now differentiate based on release monikers in the package name.

In official builds, these are different, so we should make them
different for non-official builds. Otherwise code that assumes they
are the same can sneak through and break stuff.

Binaries in Roslyn have only three possible signing states:

1. Public signed with Microsoft controlled keys (shipping binaries)
2. Real signed with a full key in the Roslyn repo (non-shipping binaries)
3. Not signed

Our build targets still had a lot of dead logic around our old methods which included delay signed binaries.  This cleans all of that up and removes some dead variables.

The specified changes don't meed the bar for Preview 4 but PR #12724 does.

- Introduce a new variable, RoslynFileVersionBase, which specifies the
  file version base we will use. This controls the file version as
  stamped in the PE, and also the version that's used for our NuGet
  packages.
- Rename RoslynSemanticVersion to RoslynAssemblyVersionBase so it's
  more clear what that's used for.
- Ensure that the PE file version header changes for each day, to give
  some protection that build-to-build upgrade correctly works for
  MSI-based installs, and also for sanity so you can have some hope
  when looking at a binary on disk.

This commit makes it so that we will always try to generate release nugets except if they depend on prerelease packages. If one of our nuget packages happens to depend on prerelease packages, then when building release nugets, we will simply print a message in the build output and skip release nuget generation for that package (and allow the overall build to succeed).