cli: add provides command

[kyrofa]

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Once the snapcraft CLI merges files from multiple parts into the staging and priming areas, it's easy to lose track of which part provided which file. This PR resolves LP: #1759700 by adding a new snapcraft provides command meant to serve exactly that purpose.

$ snapcraft provides <path>

<path> must be in either the staging or priming area, and can be a file or a directory.

[kyrofa]

Closing in favor of #2184.