Deduplicate multiple $(inherit-product) calls

The makefile product inheritance code was supposed
to deduplicate inheritance calls, but there was
a bug in the uniq-word function that caused it
to not work when duplicated words were adjacent.
$(subst |||x|||,||| |||,|||x|||x|||) produces
||| |||x||| instead of ||| ||| |||.

Rewrite the uniq-word function to fix the bug.

This issue was causing a discrepency between
the makefile and starlark based product configurations,
as the starlark implementation didn't have this bug.

Bug: 237019892
Test: ./build/bazel/ci/rbc_dashboard.py --quick on an internal-only product
Change-Id: I543a80746412ffcb9743203399413a0e707111e6