Revert "Revert "Modify symlink_forest to rerun when soong_build has changed.""

This reverts commit aa5cc2cd.

Reason for revert: Fixed the bug in the original draft.
There was an error in how we compared the file's MTime to the
file's contents.

Bug: 300288299
Test: presubmits
Test: build/soong/tests/run_integration_tests.sh

Change-Id: I9e53432c0842c0b9fc13fe20d30ce9af37640c7f