Various failures with SourceLinked files

[TheAngryByrd]

While working on https://github.com/fsharp/FsAutoComplete/pull/1007, renaming would fail sometimes. Specifically when I went to rename an override on the LspServer, I would get an error like:

Could not read file: /_/src/Server.fs

@baronfel and I suspect it's SourceLinked files, and its trying to access the Server.fs file in the LanguageServerProtocol project.

We should consider investigating when/where to ignore SourceLinked files. Goto Definition should support using them but Rename shouldn't, etc.