Recent .vsix files produced by OSS PR builds are missing files

[pjcollins]

Recent .vsix files are missing a $ReferenceAssemblies directory entirely.

Steps to Reproduce

1. Download and unzip (or install) a PR build vsix (e.g. https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android-pr-builder/2672/Azure/ ).

Version Information

PR #1356 / 8.3.99.43_HEAD_3a60f76

Log File

vsix content, missing ref assemblies:
https://gist.github.com/pjcollins/3def2556837fc42cb9690ab444c40405

[jonpryor]

Hypothesis: the problem is this line:

<ReferenceAssemblies Remove="$(LibDir)xbuild-frameworks\%(_SymLinks.Identity)\**\*.*" />

If $(LibDir)\xbuild-frameworks\.__sys_links.txt doesn't exist -- which is the case when building with xbuild -- then @(_SymLinks) will be empty, in which case this is interpolated as the empty string, which means all @(ReferenceAssembly) entries are removed.

Which is what we observe.