scoring for ambiguous reexports

[jcollins-g]

The new canonicalization fixes have some implications for packages like Angular2 which have complex
reexporting in their public libraries. Add a scorer that usually guesses right which of the libraries are really canonical.

[jcollins-g]

Promoting this to P1 as this really would make a big difference for angular I think.

[jcollins-g]

The scorer is more-or-less complete in a local branch, but I'm still working on the ability to configure it with in-line tags in the package.

Originally I thought to have each element or class tag itself which library it should be considered canonical for, but it turns out upon actually implementing that not to work so well. The problem is canonicalization depends a great deal on context.

Consider: A symbol Q defined in library X from package A reexported by another library Y in package B.

X may be canonical for symbol Q when documenting package A, but not canonical for symbol Q when documenting package B, unless --auto-include-dependencies is given, in which case symbol Q would again be canonical for X.

The approach I'm taking now is to create a @canonicalFor tag within libraries themselves. If the context means multiple libraries claim they are canonicalFor the same symbol, we can fall back to the scorer. This still doesn't cover all cases -- for really consistent behavior, we need external linkage and proper multi-package support which can enable dartdoc to always behave the same when documenting a package. Then canonicalization no longer has to be context dependent.