[catalog-server] Add Catalog.importPackage() method

[justinfagnani]

Importing a package (without a version) needs to perform a number of checks and steps:

1. Fetch the package info from the repository
2. Check if the package was imported within the refresh interval (default to something like 5 min). Abort if so.
3. Load the "packument" from the npm registry
4. Compare dist-tags and versions. If the same, abort.
5. Iterate through all changed dist tags a. Remove or add the dist-tag from PackageVersions and CustomElements as needed
6. If "latest" dist tag has changed, and the version it points to is not imported, import that package version
7. Write the new package info to the repository

[justinfagnani]

Fixed by #1329