Support external libraries

[someguy20336]

I am assuming it fixes all of the following considering I strongly suspect they are all trying to pull documentation from external libraries.

• fixes Does not work for overridden methods #1 (probably)
• fixes Problem with virtual property not being pulled through #4 (probably)
• fixes Support pulling through documentation from external libraries #9 (probably)

TODOs

• PullThroughInfo TODOs (break SyntaxNode dependency probably)
• probably more unit tests
• Can I better support external .Net libraries? The xmls don't seem to live with the dlls
• Test with VS
  • Note, this isn't working well
  • The fix doesn't show up under the quick actions. BUT it does evaluate when trying to execute another quick action.
  • My best guess is that when generating quick actions, that is run in one thread that doesn't support documentation
  • But when evaluating another fix, it is run in visual studio
  • But I cannot find ANY documentation for this.
• Make a new issue to consolidate and document some of the issues with using external libraries (commented below). However, probably close it since it doesn't seem work fixing? (issue created Outstanding Problems with external libraries #12 )
• Do I need that custom file doc provider? I dont think so
• up version
• document in readme

[someguy20336]

Things to note for when I document this:

• Xml not well supported? Unable to fetch XML Documentation from symbol a member of a custom Assembly that has a XML documentation file. dotnet/roslyn#23673
• malformed xml for < netstandard 2.1: netstandard.xml is malformed dotnet/standard#666