Create Bazel symlink forest in a separate process.

This helps with incrementality a lot: the symlink forest must depend on
almost every directory in the source tree so that if a new file is added
or removed from *anywhere*, it is regenerated.

Previously, we couldn't do this without invoking bp2build, which is
quite wasteful because bp2build takes way more time than the symlink
forest creation, even though we do the latter in a very suboptimal way
at the moment.

This means that if a source file is added or removed (which does not
affect globs), we don't pay the cost of bp2build anymore.

Also refactored symlink_forest.go on the side. Too much state was being
passed around in arguments.

This change reimplements aosp/2263423 ; the semantics of not touching an
output file is the exact same as order-only inputs and the latter is a
bit fewer lines of code.

Test: Presubmits.
Change-Id: I565c580df8a01bacf175d56747c3f50743d4a4d4