It will now be only executed for packages following the naming
convention (https://docs.gitlab.com/ee/user/packages/npm_registry/#package-naming-convention).
The validator uses Project#package_already_taken? that previously looked
for duplicates across all package type. This MR updates that function so
that it takes a mandatory argument: the package type. The duplicates
search is then done scoped to that package type

Changelog: fixed