The init.rc path selection uses the same partition_tag detection to
select an installation partition. So make sure that we always set
partition_tag even if we've been given an explicit module path.

Bug: 35314904
Test: set proprietary: true to Soong module with an init rc, verify
      that was the only change to build-aosp_arm64.ninja
Change-Id: Ic6ffd7a5177959db96a34a24896b242ee5b09e41