Skip parse/type-check of other files when modifying implementation files with backing signature files

Inverse of this: https://github.com/dotnet/fsharp/issues/10201

We should skip parsing/type-checking of other files when modifying implementation files with backing signature files; allows modifying implementation files without ever having to analyze any other file or project.