Avoid using owned `String` for package name constructors (#11768)
## Summary
Since we use `SmallString` internally, there's no benefit to passing an
owned string to the `PackageName` constructor (same goes for
`ExtraName`, etc.). I've kept them for now (maybe that will change in
the future, so it's useful to have clients passed own values if they
_can_), but removed a bunch of usages where we were casting from `&str`
to `String` needlessly to use the constructor.
