Package content tests - add `exists / not_exists`

[wolfv]

Sometimes you want to check that a file is there and sometimes the inverse.

We had ideas to use ~ foo.txt to say 'not exists' but people were against it.

Instead, let's use a dictionary like so:

package_content:
  include:
    exists: 
      - foo.hpp
    not_exists:
      - baz.hpp

[wolfv]

Note: this reflects includes and excludes from the regular "glob" places.

[minrk]

Just ran into this, and it would indeed be nice. The most common exclusion check I think I have in recipes is making sure static libs aren't being installed, or e.g. any include files or any libs when e.g. headers and things are split into multiple outputs.

[beckermr]

We need to not make arbitray changes to the yaml. Otherwise other tools won't be able to process the recipes. These kinds of changes should be submitted as a CEP.

[minrk]

Yes, I believe the CEP for this is here: https://github.com/conda/ceps/pull/84

[beckermr]

Cool. It needs to be put up for a vote and passed.