As a step towards #223, we can introduce a new table,
`external_licenses`, which provides the ability to store licensing
information for dependencies, without relying on DMD's means for
determining license data.

As part of #223, this will be retrieved from SBOMs, but this also makes
it possible to sideload the data from other sources, such as more
consistent or reliable means (taking care to note #251!).

As we're adding a new table, we need to set up the boilerplate,
including the `repositories.Repository` interface.

Note that this is slightly different to `depsdev_licenses`, as this now
includes a `package_manager`.

As these values may be inserted manually, we should make sure that
validation is enforced on the table.