bp2build: support Starlark rules and load statements.

This CL adds support to bp2build for declaring the location of the
Starlark rule definition when creating BazelTargetModules. This is
needed for non-native rules that needs to be loaded from .bzl files
somewhere in the tree.

Since load statements are aggregated at the top of the BUILD file, away
from the targets that actually use them, this CL also introduces an
abstraction to group BazelTargets together and compute their load
statements and target string representations separately, allowing load
statements to be decoupled and written into a BUILD file before the
targets themselves.

Test: soong tests
Test: TH
Test: GENERATE_BAZEL_FILES=true m nothing && build/bazel/scripts/bp2build-sync.sh write && bazel cquery //bionic/...
Fixes: 178531760

Test: TH
Change-Id: Ie5f793a00006eb024eaef07ddd9fde7aaefc054e