Remove the is_modular flag from the Package model

[dralley]

The only place it's currently used is in depsolving and retention policy handling. IIRC, it exists mostly because it was done that way in Pulp 2 more than any other reason - but this design is problematic.

1. The module upload workflow (as opposed to syncing) requires that the packages themselves already be uploaded into Pulp beforehand - but there is no way to upload a package such that the is_modular flag will be set. Therefore this is not a reliable way to know whether a package is "modular" or not.

2. Additionally we've had some problems with packages that have been synced as both part-of-a-module and not-part-of-a-module.

So, we need to ditch that field, and ensure that any mechanisms that need to know whether or not a package is modular can do so without that field being present.