Add strict mode for `package_contents` tests

[pavelzw]

As discussed in conda/ceps#57 (comment), conda/ceps#84, it would be nice if rattler-build supported a strict mode for package contents tests.

Under the strict mode, any non-matched files or any missing files would trigger an error.

The strict mode could look like:

package_contents:
  strict: true / false (default to false)
  files:
    - include/xtensor/*

With this example, any extra files would error.

[wolfv]

Hey @zelosleone - I assigned you to this issue since you are already looking at the package contents test 🙂

[zelosleone]

Hey @zelosleone - I assigned you to this issue since you are already looking at the package contents test 🙂

Perfection! Also please assign me as you like, the hardest part for me sometimes is to pick issues :D