Filesystem modules follow the installation semantic

With this change, filesystem modules (android_filesystem,
android_system_image, etc.) follow the installation semantic which means
output from modules which don't produce installable files (i.e. modules
that return false on IsSkipInstall()) are not included in the filesystem
module.

Note that this bug has been there since the beginning but didn't trigger
an error due to another bug (b/335506668) which allowed multiple
packaging specs to contribute to the same path. That bug prevented the
non-installable variant of a module (e.g. stub) from being installed
simply because because it was "luckly" clobbered by the installable
variant of the same module (e.g. impl lib).

Bug: 335506668
Test: go test ./...
Change-Id: I2a42d178701d74bf6ccc5f33ba33918f8c608d40