bp2build: flatten *_defaults properties.

This CL brings the defaults mutators into bp2build, reusing the
implementation of flattening defaults from *_defaults module deps into
the modules themselves.

This is an approach to converge defaults semantics with Bazel, which
doesn't have existing equivalent abstractions, other than macros, which
require more infrastructure to be built.

Test: TH
Test: build_conversion_test.go
Change-Id: Ia1443f8e6c3eb277fa861bb96625425a22cfc6d7