fix sourcelink url generation when deterministic paths are used on Windows

[baronfel]

Discovered this while evaluating #875, but deterministic sourcelink paths had a bug on Windows at least with path manipulation. This fixes it, let's see how CI works?

[baronfel]

A worked example:

• for the Ionide.LanguageServerProtocol package, sourcelink has an entry of "/_/*":"<github url for that repo>".
• We map that path into a regex for url substitution of the form /_/(.+).
• When going to definition, we get a url for the source file like D:/_//src/LanguageServerProtocol.fs, but the actual document.Name is /_//src/LanguageServerProtocol.fs
• Matching that document name against the regex we get a 'source fragment' of the form /src/LanguageServerProtocol.fs
• We combine this fragment with the github url from the sourcelink document to get a url of the form https://github.com/ionide/LanguageServerProtocol/raw/<git SHA>//src/LanguageServerProtocol.fs
• The // in the url result makes the Path.Combine for the temp file path bomb out on Windows, so the overall activity fails.