dartdoc should have a flag to set a non-default .packages file

[jcollins-g]

Dartdoc currently has no way to override the name of the .packages file. This can be a problem for fuchsia and other non-pub ways of organizing dart code.

[jcollins-g]

This turns out to be useful for monolithic repositories too. Increasing to P2.

[devoncarew]

@bwilkerson, who has knowledge of the Fuchsia / GN resolution scheme. For dartdoc to work for Fuchsia, I imagine it'll need to understand that resolution scheme.