Remove buildtools imports from Directory.Build.* (dotnet/corefx#34001)

Remove buildtools imports from Directory.Build.* (dotnet/corefx#34001)

* Remove buildtools imports from Directory.Build.*

Remove buildtools imports and implement remaining functionality in CoreFx.

I was able to do most of what BuildTools was doing with 3 exceptions, which will be split into separate issues.
1. Test shared framework deps file: GenerateDepsJson.  We need to port this task to arcade or change how we test to not need it.
2. codeOptimization.targets -> should be moved to arcade, work is pending.
3. optionalTooling.targets -> move to a CSProj that is restored by the official build workflow.

* Address feedack, fix unix and UAP build issues

* Fix UWP, Tizen, and AllConfigurations build

* Remove buildtools dependency from package testing

* Remove a few more buildtools dependencies

* Eliminate RunningOnUnix property

* Remove NuGetTargetMoniker from shims.proj

* Remove ResourcesFolderPath

* Fix Microsoft.XmlSerializer.Generator.Tests

* Specify NuGetTargetMoniker for shims

* Remove a couple remaining ToolsDir uses

Commit migrated from https://github.com/dotnet/corefx/commit/49d249fa023e8575dba7e35febb4446e4fa13f98