Add scoped lookup to Libraries

[jcollins-g]

This adds a scope implementation for Library which allows us to use the analyzer scope for imported and defined objects, reducing the custom implementation to allowing export lookups. Since we prioritize analyzer resolution, edge cases will now favor imports over exports where it can make a difference.

Also did some small cosmetic changes to make warning links show up more consistently in IntelliJ terminal output.

Comparison of sdk docs using dart --enable-asserts dartdoc.dart --sdk-docs --warnings reference-lookup-found-with-new,reference-lookup-missing-with-new,reference-lookup-not-found-with-new --show-stats:

this PR:

total references: 29582
resolved references:  22262 (75.25522277060375%)
resolved references with new lookup:  28738 (97.14691366371441%)
resolved references with old lookup:  22262 (75.25522277060375%)
resolved references with equivalent links:  28996 (98.0190656480292%)

Head:

total references: 29582
resolved references:  22262 (75.25522277060375%)
resolved references with new lookup:  26672 (90.16293692110067%)
resolved references with old lookup:  22262 (75.25522277060375%)
resolved references with equivalent links:  26945 (91.08579541613143%)

[srawlins]

total references: 29582
This PR: resolved references with new lookup: 28738 (97.14691366371441%)
Head: resolved references with new lookup: 26672 (90.16293692110067%)

That's wild! Huge boost.

[coveralls]

Coverage increased (+0.02%) to 57.566% when pulling bd8d3c5 on jcollins-g:lookup-test-iter into 02a467b on dart-lang:master.

[srawlins]

👍