change how builds/arches work

- builds are downloaded to
  ~/.meteor/downloaded-builds/foo/1.0.0/browser+os
- but this is not where packages are loaded from
- instead, we "merge" multiple packages together and move them into
  ~/.meteor/packages

This commit does not yet implement "oh, we need another arch now?
ok. download and merge it."  Well, it implements "download it" but not
"merge it".

This drops catalog.getAnyBuild and a reference to "browser+os" in
packages.js, yay