Simplify logic of determining Reference definition

[TylerLeonhardt]

fixes PowerShell/vscode-powershell#2374

Before, I had the logic backwards and the CodeLens would only show 1 reference (the reference definition!).

After, now we only check file path and line number. Simpler logic.

I added a TODO to think about what would happen if we find two of the same named function definitions:
fileA

function Test-Foo { }

fileB

function Test-Foo { }

The references will probably show up... but I think there's not much we can do since the symbol reference doesn't seem to give us anything on whether it's a definition or not which is really a shame.

[rjmholt]

LGTM, although would like to discuss more how we can refine/improve our symbol logic