Implement a "include_files" or "files" key in build

[wolfv]

As a stepping towards "split-builds" from a cache, we should implement a files key in the build section.

This key would contain a list of globs, and only the matched files would be included in the package.

That way, someone could, for example, create a package with only the header files and so on.

E.g.

build:
  script:
     - bla bla
  files:
     - include/*

It might be good to have some post-processing step that would warn if some files were "forgotten" to be included in any of the outputs (although that really only makes sense once we have the cache step).

[wolfv]

implementation started in #819