provide access to a package iterator for ebuild repos that validates/regenerates metadata in parallel

[radhermit]

This goes along with adding support in pkgcraft for detecting, configuring, and/or flagging repo types as read/write vs read-only being that read/write repos have to validate and update metadata (to be safe) while read-only repos don't.

For read/write repos, we should be able to provide access to a custom iterator type that regenerates metadata in parallel in the background on demand as necessary while serially returning the next package with valid metadata.