Move target path calculation out of MSBuild

[chsienki]

We should move the target path calculation from MSBuild into the source generator always. We have the project file passed in, so the calculation is trivial to do in the generator directly.

My suspicion for why it was done like this is that the old pre-SG way of compiling had logic in MSBuild for target path calculation, and the authors didn't want to have a duplicate that could get out of sync.

Originally posted by @chsienki in #12355 (comment)

[chsienki]

This would also make setting up tests easier, as we wouldn't have to provide the target path for every file via an editor config.

[davidwengier]

I would love to see this, but we need to double check things like linked files. I don't think the LSP editor supported them, but the source generator probably did if the target path was set right, and not sure Roslyn exposes any information that the source generator could use to calculate them.

This would also make setting up tests easier, as we wouldn't have to provide the target path for every file via an editor config.

I suspect the PR this issue came from fixes that already :)