This reverts commit d43d4780.

Right now there are two code paths for signing using a keyfile:

1) Full & delay signing implemented using a strong-name provider, which
   calls into the CLR when necessary to do signing.
2) Public signing, which extracts the key directly from the file and
   embeds it manually into the assembly using only portable code.

The tricky part is that (1) also implements a bunch of legacy semantics
on how to resolve the key file from various search paths, since the key
file can also be specified via an attribute.

This behavior is already deprecated, so rather than implement the
complicated file resolution for public sign as well, this PR changes the
command line compilers to always produce an absolute path for the key
file before passing it to CompilationOptions. CreateCompilation now also
has a check to enforce that CryptoKeyFile is an absolute path when
PublicSign is set.

Fixes #8360.

This reverts commit 57a34148.

This reverts commit 760e1204.

This reduces package restore on Roslyn.sln with all packages already restored from 40 seconds -> 10 seconds.

project.lock.json format has changed since we last updated.

This reverts commit e2fc5135.

This reverts commit e3c1ea08.

This reverts commit 8f26ffb5.

This reverts commit 662593ba.

This still only runs the tests on desktop (for now), but it makes
it possible to run them on coreclr.

Synchronize all corefx references to -23504 and bring in the
xunit coreclr test runner.