Generate a all_modules.txt file on every build that
can be used for autocompleting.

Because this adds time to every non-incremental build,
I profiled it by using `$(shell date +"%s.%3N")` before
and after, and disovered it only adds about 0.01 seconds
to the build. And of course it won't be rerun if there
weren't build changes.

Bug: 255867758
Test: Manually
Change-Id: I1d3f1008611fd542884322b27b19ea2fa1d341fe