Allow Configuration of Path to Swift Compiler for SwiftPM

[RLovelett]

Currently there is a method findBinDirectory in Server.swift that attempts to find the path the Swift compiler.

This compiler path is used by SwiftPM to interact with the Package.swift. Currently, if this path is not found or incorrect it causes a fatalError and hard crash of the language server.

I'd like to do (at least) two things before calling this issue done.

• Allow the path to be configured/set by the user and directly passed in initializationOptions as part of InitializeParams.

• If for some reason, the path is invalid or the compiler cannot be found, send an error to the client notifying the user that the path is invalid. Rather than crash.

• This one is a stretch goal. Modify library_wrapper.swift to make use of this path instead of the existing logic in place.