Preserve depset structure from bazel aquery

Each depset now corresponds to a phony rule which depends on other
depsets or on full paths; thus, bazel's depset structure is preserved in
the form of phony rules of name bazel_depset_{id}.

Previously, flattening and recopying large lists of file path strings
was quite inefficient. This was particularly evident as we enumerated
hundreds of clang headers for each cc compile action.

This reduces soong_build analysis time by about 30% for mixed builds.
It also reduces ninja file size by ~750MB.

Fixes: 229405615
Test: Unit tests, manually verified metrics, mixed_droid CI

Change-Id: I78df152ac1488ae0c6807afdde4b4ad5e6d26287