[WIP] Add call to FSharp.Editor bits in the legacy project system

[cartermp]

#4108 wasn't complete, apparently.

• The legacy project system was still calling into the FSharp.LanguageService version of ProjectSitesAndFiles.fs via fully-qualified name
• Parts of the project system tests were doing this as well

These are now updated to call into the correct things, thus ensuring that F# source files from .NET Framework-based projects aren't rendered as plaintext.

.NET Core SDK-based projects and VisualFSharp.sln all seem to load and work with these changes.

I did notice, while testing, that automatic restore for .NET Core SDK-based projects wasn't happening.

TODO

• Find any further usage of FSharp.LanguageService.ProjectSitesAndFiles in the tests and replace then with the FSharp.Editor version
• Potentially add a test to see that once a file is opened, the lang service is initialized

[cartermp]

Okay, so auto restore being borked is definitely unrelated to this

[cartermp]

Closing due to rebase being dumb