Fix failing tests and path parsing on Windows

[radeksimko]

Closes #71
Closes #72

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Tested in a Windows VM:

[paultyng]

This LGTM for local system paths, we'll probably need to talk about UNC paths at some point, I'll open an issue to investigate them, see if I can come up with a repro.

[radeksimko]

Agreed, also the implementation is very similar to gopls’ one, but the main difference is that we detect Windows-style paths based on where the LS is running (build flags), which technically isn’t correct, as the OS of client and server may differ (i.e. Unix-built LS may receive Windows-style paths), but there are other parts of LS which don’t account for this eventuality anyway, so I can address that also in a separate issue/PR.

[ghost]

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