require fully specified comment references rather than allowing partial lookups via canonicalization

[jcollins-g]

The old lookup code will permit users to write expressions that can only be evaluated in the context of the dartdoc-canonical libraries in which they are documented, which may have a synthetic namespace constructed via reexports. This is not a good idea for implementing support for resolution in software besides dartdoc.

[srawlins]

Closing this in favor of just removing the reference lookup system.