Might fix https://github.com/dotnet/roslyn/issues/28782

This version includes now required `signers` entry to Willow package manifest.

This includes the full set of net461 reference assemblies.

* Move VS specific targets to VisualStudio.targets

* Move optimization targets to RepoToolset/OptimizationData.targets

* Move SourceLink targets to RepositoryInfo.targets

* Remove workarounds for fixed issues

* Move workrounds to Workarounds.targets

* Clean up binding redirect generation properties

* Remove unused targets

* Normalize PushToBlobFeed parameters to values used by RepoToolset.

* Use System.Runtime.Loader light-up package

* Only import package versions in projects that restore toolset packages

* Add nuget.org feed

* Use Microsoft.SourceLink.GitHub 1.0.0-beta-63001-01

* Workaround XAML bug

* Fix CommitHashAttribute

* Update compiler package to 2.9.0-beta7-63018-03

* SymLink .git directory for 2nd deterministic build

* Orchestrated build dependency uptake

* Remove duplicate NuGetPackageRoot definition

Update XliffTasks to the latest version. Notably, this version produces
better MSBuild error messages when a .xlf file can't be loaded as XML.

This updates the resource DLL which corrects the version of mscorlib for
the net45beta resource.

Fixes https://github.com/dotnet/roslyn/issues/23424

Allows the compiler extension to work with CPS based projects

Update to the latest version of XliffTasks. This version has fixes for the targets for sorting .xlf files and checking that all resources are translated so that they work properly on projects targeting multiple frameworks.

This copies our VSTS YAML file into dev15.7.x. This changes nothing
about our official build in this branch. It just seeds it with the
correct value and lets the merge bots move it to the rest of the trees.

The official switch over for 15.7 will happen at a later time and will
be very deliberate.

This reverts commit ea88bb13.

Update to the latest version of XliffTasks. Notably, this version
attemps to insert new `trans-unit` elements into .xlf files in a sorted
order rather than always putting them at the bottom. This should help
reduce merge conflicts.

This designation will allow Visual Studio to kill instances of
VBCSCompiler rather than showing it as blocking update installation.

closes #24610

Need to generate a binary log during signing so we can understand why
our builds are failing when new signing is enabled.

This fixes issues where CreatePkgDef needs to get a long command line
longer than the usual Windows limit.

This change moves the MSBuildWorkspace tests to a separate project and introduces infrastructure to get them
running. Essentially, each test is marked with a `ConditionalFact` with `VisualStudioMSBuildInstalled` as the
skip condition. That skip condition uses the VS 2017 setup APIs to locate installed instances of VS and finds
one that contains the Roslyn Compiler package. Then, it sets up an AssemblyResolve event on the current AppDomain
to resolve MSBuild locations from VS itself. This ensures that MSBuild can locate a toolset containing the
necessary props and targets to handle the projects in our tests.