PMC initialize should cast DTE2 on main thread (in service locator)

[zivkan]

Bug

Fixes: NuGet/Home#11320

Regression? No

Description

Request DTE2 from the service locator, rather than getting DTE and then casting on the current thread.

PR Checklist

• PR has a meaningful title

• PR has a linked issue.

• Described changes

• Tests

  • Automated tests added
  • OR
  • Test exception
    • This is a thread timing bug when something calls a NuGet API from the UI thread before VS's NuGetPackage is initialized. To me, this feels too obscure to create a test for, as there will be a very high false-negative rate, and in the case where the scenario is hit, it's difficult to cause the test to have a failed result, rather than timeout.
  • OR
  • N/A

• Documentation

  • Documentation PR or issue filled
  • OR
  • N/A

[zivkan]

Bypassing merge policy, because this issue is getting attention from high up, so we need to get this in. The build failure is due to infrastructure issues, not product changes (this change doesn't even run on Mac)