monolithic repository mode for dartdoc

[jcollins-g]

Dartdoc needs to work where there is one big repository holding all Dart code, like at Google. This meta-issue will track enhancements required to make this function.

• Allow running from Dart code. #1609 - Allow running from Dart code instead of fork/exec
• Refactor so that docs are generated through the Package class #1659, Refactor DartDocConfig object usage, part 1 #1661, Refactor DartDocConfig object usage, part 2 #1662, Document arbitrary SDK-like things as Dart SDKs. #1665 - Refactor of Package/PackageGraph into LibraryContainer, Package, Category, and PackageGraph. Necessary to be able to implement a new LibraryContainer for directories in the One Big Repo.
• link to identifiers in sdk on api.dartlang.org from third-party package #739 - Support for cross-linking between packages.
• Implement a LibraryContainer / Package subclass that is a single directory #1670 - Support "packages" that are single directories in a monolithic repository.
• dartdoc does not obey analysis_options files #1672 - Dartdoc does not obey analysis options files