Expose packages in `PackageResolver`

[andreaphylum]

Currently, the Project constructor requires users to specify the list of the packages that are available in the resolver, in addition to supplying the resolver itself.
We should add support for enumerating packages in PackageResolver in order to remove the redundant argument passed to the Project::new constructor.

let package_resolver = PackageResolver::builder()
  .with_package("path-scurry", Package::from_tarball_path("./tarballs/path-scurry-1.6.1.tgz"))
  .with_package("lru-cache", Package::from_tarball_path("./tarballs/lru-cache-7.14.1.tgz"))
  .with_package("minipass", Package::from_tarball_path("./tarballs/minipass-4.0.2.tgz"))
  .build();
  
let project = Project::new(
  package_resolver,
  vec!["path-scurry", "lru-cache", "minipass"]
);