reduce forest generation to be incremental

Previously, symlink forest generation involved removing the entire
symlink forest and recreating it from scratch. With this change,
a) symlinks which need not change are untouched,
b) symlinks pointing to the wrong location are fixed, and
c) symlinks which should no longer exist are removed.

On AOSP on my local machine, this reduces the symlink forest generation
step from 2.5s to 1.1s clean, and 0.6s when a single file is added to
a source directory.

Bug: 257528847
Test: m bp2build, touch `fakefile` under the forest, remove a file
from the source tree, rerun m bp2build. Manually verify the new forest
does not retain the link to the deleted source file, and that fakefile
no longer exists in the forest.

Change-Id: I481371ae487e9419af6a3a4370c552578b07d650