Fix issues with active document on other features

When I open a new document, this assertion https://github.com/fsprojects/VisualFSharpPowerTools/blob/f4714a54d08ebeaaf8c9f43206cda219efbea579/src/FSharpVSPowerTools.Logic/ProjectFactory.fs#L135-L136 fails for all smart tags features. `GetActiveDocument` is incorrect for the first moment we check for it.

To ensure that the current document refers to the text buffer, we use file paths to do sanity check. It's an intrusive change, but I don't know how to limit its scope further.

Perhaps I was too obsessive. Will remove that and add an assertion on null check instead.