* 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

From beta6 to beta7

Directory.Build.props and Directory.Build.targets are handled by the
project system, so are not included in this change.

Ideally, we'd apply a version number to our binaries like 20180102
for the 2nd of January, 2018, but the Windows PE file format restricts
each number to a 16-bit unsigned integer. This means even trimming
to 80102 still isn't enough. The solution is to continue to decrement
that 'year' digit, and each time we do so add another 12 to the month.
This buys more time until we can rebaseline where we start.

This is incrementing our version numbers as a part of moving to the next VS release.

Add a blank line in Versions.props back for nicer formatting

since this branch is now targeting Preview 4.

* Update `RoslynAssemblyVersionBase` from 2.3.0 to 2.6.0
* Update `RoslynFileVerionBase` from 2.3.0 to 2.6.0
* Reset `RoslynNuGetMoniker` to beta1

Also incorporate @shyamnamboodiripad's fix in #19546.