- Prior to this if a 15.8 VS was installed the VSIX creation target would fail due to a bug in the CLI we're currently pinned to (already fixed, we just haven't pulled it). The fix was removing the parallelism flags to msbuild to allow it to run unrestricted in regards to processor utilization.
- Changed the VS restrictions to ensure we don't use unsupported 15.9 bits that have breaking changes in their MSBuild bits (this has proven to be troublesome in the past).
Also did some spring cleaning on redundent references in the language
services package.
Note that the 'immutable' packages are now totally redundant with
Shell.15.0. You're supposed to use one or the other. Since our minimum VS
is 15, I just went with shell 15.0.
Now the VSIX project doesn't have many references in it.