Changes:
1. Require `blueprint-compiler` 0.16.0 (or newer) for building from
a git checkout. With #6822 distributions that can't meet that
requirement can use generated source tarballs to build.
2. Remove all `.ui` files as they are unnecessary.
3. Simplify the `Builder` interface since raw `.ui` files are no
longer used.
4. Removed build-time check of raw `.ui` files.