Storage service: refactor out the use of lsar in get_base_directory()

[alexwlchan]

Please describe the problem you'd like to be solved
Currently the storage service uses lsar to find the base directory of a compressed AIP, code here: https://github.com/artefactual/archivematica-storage-service/blob/83d7b5a7da79c158cb99bd0e3426b92fdde0d3f0/storage_service/locations/models/package.py#L343-L361

This code is potentially inefficient – it uses more and more memory as the size of the AIP grows (both in the output from lsar, and the size of the directories list). It might struggle on a very large AIP.

Not a bug per se, but potentially room for improvement.

Describe the solution you'd like to see implemented
We know an AIP will only be compressed in a handful of formats (because Archivematica created them!). Use the Python standard library to open the file directly, and iterate over the members, rather than loading it all into a big JSON string.

Pseudo-code:

for member in get_members_of_archive():
    if member.is_directory() and len(member) < len(shortest_dir):
        shortest_dir = member

return shortest_dir

There's already some code to identify compression formats in utils.py.

Describe alternatives you've considered
None.

Additional context

• Related: Problem: 7zip and unar deps are overlapping artefactual/archivematica-storage-service#191. Handling the compression in the Python standard library rather than shelling out means one less dependency.
• I discovered this while investigating Archivematica is currently failing to start a reingest wellcomecollection/platform#4004, in which this code is failing to find any directories in the downloaded AIP (because the tar.gz doesn't have any directory entries).

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