soong: Add PathForSourceRelaxed

Used by vendor/lineage generated kernel header module.

Partial pick from:
Author: Sam Mortimer <sam@mortimer.me.uk>
Date:   Fri Aug 17 11:25:08 2018 -0700
    soong: Add java sources overlay support
    Change-Id: I94143febb0a8afa6a165364d36a40d5120a4e7bc

Change-Id: I415af71458f2a7be8e256cb3c548994f09c5bebf