Support unnamed requirements in `--require-hashes` (#2993)
## Summary
This PR enables `--require-hashes` with unnamed requirements. The key
change is that `PackageId` becomes `VersionId` (since it refers to a
package at a specific version), and the new `PackageId` consists of
_either_ a package name _or_ a URL. The hashes are keyed by `PackageId`,
so we can generate the `RequiredHashes` before we have names for all
packages, and enforce them throughout.
Closes #2979.
