Put BuildDetails.cs in ATxCommon instead of having it shared.

The shared approach results in conflicts if the generated class is used
in ATxCommon which is imported in the other projects. This way we can
use the symbols defined via ATxCommon in all depending projects.

The downside is that it slows down the build-process a bit as every
build from a project depending on ATxCommon (read: all others) will
change the generated code and thus force a re-build of ATxCommon and
the project itself. Since building the entire solution is still just
a matter of seconds, that's a fair trade-off.