Provide a way to exclude packages/slices when running cut

[lengau]

It would be nice to be able to exclude certain packages or slices when running cut, even if those are dependencies of the requested chisel slices. This would allow snaps and charms to be able to use chiselled packages without pulling everything all the way back to libc.

Ideally for my particular use case (integrating chisel into more craft* apps), we'd be able to pass an exclusions file (one package/slice name per line in a text file), and if a slice is explicitly listed it overrides that exclusions file.

Examples for why I'd like this

A core24 snap that only stages `curl_bins` is 8.6 MiB, whereas using craft-parts's ignore list but extracting the full debs is still only 1.1 MiB. Here's the `snapcraft.yaml` file:

name: my-snap-name # you probably want to 'snapcraft register <name>'
base: core24 # the base snap is the execution environment for this snap
version: '0.1' # just for humans, typically '1.2+git' or '1.3.2'
summary: Single-line elevator pitch for your amazing snap # 79 char long summary
description: |
  This is my-snap's description. You have a paragraph or two to tell the
  most important story about your snap. Keep it under 100 words though,
  we live in tweetspace and your description wants to look good in the snap
  store.

grade: devel # must be 'stable' to release into candidate/stable channels
confinement: devmode # use 'strict' once you have the right plugs and slots

parts:
  my-part:
    # See 'snapcraft plugins'
    plugin: nil
    stage-packages:
      - curl_bins  # Remove _bins here to extract the full packages

A real-world example that could use this feature are the postgresql charms. Currently they're using a workaround because Charmcraft doesn't stage packages correctly. However, an "ideal" version of this would have this part instead:

libpq:
  plugin: nil
  stage-packages: [libpq5_libs]