Enhancements:
-------------

* detect "used" variables in build.sh, bld.bat and any output scripts, in
  addition to what already existed in meta.yaml. Used variables end up in the
  hash. #2576
* don't merge build and host prefixes. We used to do this when host subdir ==
  build subdir. Keep them separate, so that build tools in build prefix won't
  ever interfere with software installed to host, to be packaged.  #2579

Bug fixes:
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* exclude grouped keys from zip_keys when computing hashes.  Only direct dependencies affect the hash.  #2573
* fix always_include_files usage omitting other ordinarily installed files  #2580