feat(pack): automatically install non-matching charmlibs

[lengau]

If charmcraft.yaml specifies charmlibs, pack will now automatically upgrade libraries on disk that don't match the specifications in the yaml file.

[sergiusens]

@jdkandersson ^

[sergiusens]

I copied verbatim from the spec, which believe we are not following with the change as described

---

The library fetching code can now always be run as part of charmcraft pack. There are three possible options during the pack process:

• The required libraries are not in the source tree, and are fetched as part of the packing process
• There exist libraries in the source tree that satisfy the requirements
• There exist libraries in the source tree that conflict with charmcraft.yaml requirements (or are not specified at all)

It is important to note that while charmcraft pack will now be looking at the store, the charmcraft pack should never mutate the libraries on the filesystem, except in the first case. The interaction with the store during the pack process is only to provide warnings or information about the libraries in use (such as the availability of updates). The inability to reach the store should not make the pack process fail (except in the first case).

In the second and third cases, the fetch code is still run - if there are newer versions of the library available in the store, or the libraries do not match the constraints specified, the user should be notified as part of the packing process, but it should only be a warning/info notice, and the pack process should carry on using the in-tree libraries.

[sergiusens]

Looks good, have some minor comments and perhaps an expectation mismatch on fetching on pack (I thought it would be ephemeral and not persisted if I ran charmcraft clean)