Add depfile support to RuleBuilder

Allow rules built with RuleBuilder to use depfiles.  Ninja only
supports a single depfile with single output.  If there are
multiple outputs in a rule, move all but the first to implicit
outputs.  If multiple depfiles are specified, use new support
in dep_fixer to combine additional depfiles into the first depfile.

Test: rule_builder_test.go
Change-Id: I7dd4ba7fdf9feaf89b3dd2b7abb0e79006e06018