Introduce Advisories to track arbitrary package advisories

As a way to provide a better means to annotate certain dependencies with
information about why the package(s), or certain versions of a package
must not be used, we can introduce the concept of Advisories.

This produces a new table, `advisories`, that can be used to insert
arbitrary advisories for package, such as "this package is no longer
maintained" or "don't use this because of a security vulnerability".

We can add a `db generate advisory` subcommand which can pre-seed with
some community-provided package advisories, as an example of what can be
done.

We can use a more complicated query to perform this lookup using a
`case / when` which allows us to perform more logic inside our query to
look up the data, instead of performing the lookup in Go code.

This uses an enum-style approach inside the database schema to provide a
bit more control over the entries, as this is more likely to have custom
SQL executed against it, so we want to make sure that we're not
going to end up with out-of-sync data.

Unfortunately sqlc seems to be unhappy when using multiple `INSERT`s, so
we need to execute the query manually.

Closes #45.