fix: don't generate advisories for Drupal distributions (#148)

Distributions are not sourced through Composer so these advisories
currently don't map to an actual packages and are getting flagged by the
`osv-linter`.

Some of these advisories do seem to name specific modules which might be
available via Composer, so these could come back in future but that'll
require having the specific module represented in a machine-readable way

Author: G-Rath