Support redirecting full URL to latest

[kevmoo]

The build readme files links to classes using latest in the URI

See the readme

[dartdoc:Builder]: https://www.dartdocs.org/documentation/build/latest/build/Builder-class.html
[dartdoc:BuildStep]: https://www.dartdocs.org/documentation/build/latest/build/BuildStep-class.html
[dartdoc:Resolver]: https://www.dartdocs.org/documentation/build/latest/build/Resolver-class.html

As is currently deployed

https://pub.dartlang.org/documentation/scratch_space/latest/scratch_space/ScratchSpace-class.html

redirects to https://pub.dartlang.org/documentation/scratch_space/0.0.2/

It should redirect to

https://pub.dartlang.org/documentation/scratch_space/0.0.2/scratch_space/ScratchSpace-class.html

FYI: I'd rather have #1017 fixed so that https://pub.dartlang.org/documentation/scratch_space/latest/scratch_space/ScratchSpace-class.html
is the actual URL for the current release – but that's beside the point