Replace `SymbolMatch.BaseURI` with new `result.File`

Created by: camdencheek

This commit replaces the information stored in a SymbolMatch.BaseURI with a reference to its containing file.

The goal of this is to be able to replace the call to SymbolMatchResolver.URL() with a SymbolMatch.URL(), but currently, a SymbolMatch does not contain the information needed to create that URL.

In order to add a reference to its containing file without introducing circular references between result.FileMatch and result.SymbolMatch, the information identifying the file itself was pulled out of result.FileMatch into a new result.File and was added to the result.SymbolMatch struct.

Additionally, this commit removes the gituri package. The only place it was being non-trivially used was in SymbolResult.BaseURI, and that no longer exists.

Thanks @keegancsmith for pairing on this!