Modify symlink_forest to rerun when soong_build has changed.

Also remove existing symlink_forest_version functionality.

This remedies issues pertaining to b/300122962 - symlinks not
clearing after a revert

Timing wise - this doesn't present a performance regression
on a clean build. When soong_build changes, it's considerably longer
but no different from the time when symlink_forest_version is changed.

Bug: 300288299
Test: build/soong/tests/symlink_forest_rerun_test.sh
Change-Id: I0e95aac315dfea7ea3b8bb9a3eb0c6408300bd3b