Correct links to remote, canonical libraries

[srawlins]

Fixes #3891

In this example, the file package exports elements from dart:io. So when --link-to-remote is used, the canonical library for those exported elements should be api.dart.dev.

Also some cleaning:

• Move _searchForCanonicalLibrary from ModelElement to canonicalization.dart, keeping more canonicalization logic together.
• This allows us to make Canonicalization private.
• Make some comments in ModelElement doc-comments.

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[srawlins]

Ready, @dart-lang/analyzer-team