repo_management/defaults.py:
Add the IntEnum `RepoDbType` to be able to identify/address the
different types of binary database files (e.g. .db vs. .files).
Change the %LICENSE% entry in `DESC_JSON` to not track the name in
plural (use "license" instead of "licenses") to stay in line with the
naming of the original variable.
Add `DB_USER`, `DB_GROUP`, `DB_FILE_MODE`, `DB_DIR_MODE`, to provide
defaults for which user and group to create files and directories with
which file mode for, when generating a database file.

repo_management/models.py:
Change the `License` model to track an optional list of license
identifiers using the singular term ("license" instead of "licenses") to
have more predictable and coherent naming of variables and attributes.
Add `Base` to `OutputPackageBase` as this way the model is more complete
and it becomes much easier to use it, without having to also pass the
pkgbase alongside it.
Add the convenience method `get_packages_as_models()` to
`OutputPackageBase` which returns the list of packages as tuples of
`PackageDesc` and `Files`.
Extend `PackageDesc` by the convenience method `get_output_package()` to
easily convert an instance of `PackageDesc` and an optional instance of
`Files` to an instance of `OutputPackage`.
Move the declaration of `OutputPackage` above that of `PackageDesc` so
that `PackageDesc.get_output_package()` can declare typing.

tests/test_models.py
Add tests for `PackageDesc.get_output_package()` and
`OutputPackageBase.get_packages_as_models()`.