Resolve ThisProject properly

Fixes #1194.
Since `0.13.1` and possibly earlier, `ThisProject` resolves to the root project in a build. 
That is because the resolution has two steps:
* `replaceThis`: substitutes `This` with the correct value across all axes, then
* substituting `Select(_: Reference)` (the project/build selector, i.e. `scope.project`) with a resolved reference to whatever is referenced

But by the time the 2nd step runs, it does not have enough information anymore to determine what the current project was, because that information is not passed to `resolveProject` or `buildResolve`.

My solution is to transform `Select(ThisProject)` into `This` before step 1, so that `replaceThis(thisScope)` picks it up and resolves it first.