Draft: Adds warnings if automounts seem to be required

[freider]

Proof of concept of warning when packages from global scope imports haven't been added to an image using .add_local_python_source.

This could be used in a longer transition period towards deprecation of "auto mounting".

There are a few things that need to be fixed for this to work well:

• Iron out the details of how what kind of "entrypoint" mounting we want in both script and module notation modal run/deploy:
  • For example, modal deploy my.pak puts my.pak in sys.modules, and it's not 100% clear if we want it to automatically mount everything in my or just the directory structure + init files required to import my.pak.
  • Even something like modal run myscript.py will actually put myscript in sys.modules as a top level package which would get automounted (but currently deduplicated)
  • If you have a package but still use a "script path" within that package as your modal entrypoint, you risk getting two copies of that module: one at the top level /root/script.py and one in your module structure: /root/mypak/script.py and side effects on the module level would be different between the two - potentially causing bugs. These files should preferably be deduplicated in favor of the module variant, but that isn't done today.
  • There is currently a bug with mount deduplication when using relative paths to refer to imports, e.g. modal run ../foo/bar.py
• Add a way to specify automounting behavior on an @function (or app) level, to let users silence this warning without resorting to global config/env vars
• Should we allow explicit auto-mount = on values in the future as well, with all the foot guns that entails? (at least during a transition period it could be nice to have that as a fallback for users once we turn on auto-off)

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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