Missing PURL converters

We've added a few [OSV ecosystems](https://github.com/google/osv.dev/blob/260a49adcac4d7d06d75b2f53c0915225e15518e/osv/ecosystems/_ecosystems.py#L33) without adding [PURL conversions](https://github.com/google/osv.dev/blob/260a49adcac4d7d06d75b2f53c0915225e15518e/osv/purl_helpers.py#L18) for them.

Ecosystems that don't have corresponding PURL entries (And their [known purl type](https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst)):

- [x] `AlmaLinux`
- [ ] `Android`
- [ ] `Bioconductor`
- [x] `Bitnami` (`bitnami`)
- [x] `Chainguard` (`apk/chainguard`)
- [x] `ConanCenter` (`conan`)
- [x] `CRAN` (`cran`)
- [ ] `GHC`
- [ ] `GitHub Actions` (`github`?)
- [ ] `Linux`
- [ ] `Photon OS`
- [x] `Rocky Linux`
- [x] `SwiftURL` (`swift`) (thanks @sschuberth!)
- [x] `Ubuntu` (`deb/ubuntu`)
- [x] `Wolfi` (`apk/wolfi`)

Some of the OSV records have PURLs already because the data source provides them (e.g. Bitnami, Chainguard & Wolfi)

I think we need to come up with some way to make sure we always have these, and explicitly define ecosystems with no PURL equivalents.