Support StoreKit Configuration setting in xcscheme Run action

[aaronsky]

Fixes #3130.

Redoes #3182 (and potentially fixes unresolved comments on #3185).

The StoreKit Testing configuration file has a peculiar requirement that has no precedent in the rules. The file's path in the scheme must be relative to the xcshareddata/xcschemes directory the scheme is under, not the scheme itself (so one level shallower than you'd expect). Making the path relative to the project fails, and making it absolute crashes Xcode.

A prior iteration of this PR attempted to relativize the path in Starlark, but exposed some safety shortcomings. In knowing the absolute path of both the scheme directory and the configuration file, doing this work in the generator gives us slightly improved safety guarantees at the cost of performance. To this author, there is too much ambiguity between the paths of either location without this information, since the files can potentially exist in independent file trees under a workspace.

[aaronsky]

@brentleyjones I moved the path relativization back into the generator, and updated tests. I couldn't find a good heuristic to use as an anchor in the Starlark, whereas we have the full workspace path in the generator. Hopefully this doesn't result in an intolerable regression of project incrementality. Please take another look at your soonest convenience.

[brentleyjones]

Please add tests for ‎tools/generators/xcschemes changes, or minimally the relative path logic part.