Make BUILD file merging slightly smarter.

This change enables checked-in BUILD files like
prebuilts/clang/host/linux-x86/BUILD.bazel to be merged cleanly with the
bp2build generated one into the synthetic workspace.

The checked-in BUILD file contains a package() declaration that bp2build
also generates. To avoid double declaration, the BUILD file writer now
checks if the BazelTargets contain handcrafted targets. If so, it
delegates the package declaration to the handcrafted BUILD file instead.

This change also sorts the bp2build targets before the handcrafted ones,
and adds a section header to demarcate the two sets of targets.

Test: TH
Change-Id: I3ecdeaab3226b895b623daf0791d24a657f7a7c6