Change symlink_forest to use relative symlinks.

Also add script to remove the Bazel output base.
This will assist with supporting movable checkouts alongside
mixed builds.

Bug: 259191764
Test: m && (move topic and prepare_moved_top.sh) && m
Test: m && prepare_moved_top.sh && m
Test: build/soong/tests/relative_symlinks_test.sh

Change-Id: I0f53da8d99f752fad496cf3ac61b01f001b7296d