* [XamlC] Better XF .targets errors reporting
- throw build errors if multiple xf .targets are imported, or if the
.targets doesn't match the Tasks.dll ABI. That validation is skipped
when building in CI, as the build task is not yet present at
validation time.
- reuse the main .targets for UnitTest builds, avoid duplication
- parameterize the .targets
* define defaults before import
cc2129395b