Adds support for custom Packagist registries.

- [x] Support dynamic packages.json interface for wpackagist.org
- [x] Support static packages.json for satis
- [x] Support basic auth using hostRules
- [x] Write basic auth to `auth.json`

Closes #2574, Closes #2354 

Renames "date" and "time" values from datasources to instead be "releaseTimestamp".

This reverts commit 29d00aa4.

Adds a wrapper to all datasources to provide a generic purl-based interface.

This PR adds the packagist datasource plus basic exact semver Composer support. Composer lockfile updating is not yet supported.