canonicalization warnings for external libraries should be silenced by default

[jcollins-g]

Another part of #1412 is that we presently don't treat public libraries from other packages any differently than private libraries within the same package. Make that distinction inside dartdoc, and silence canonicalization warnings for things that are outside the documented package.

This distinction existing in the code is a prerequisite for supporting external linkage.

[jcollins-g]

#1524 fixed this.