Ensure that all testsuite dependencies are distributed.

Failing to distribute all testsuite dependencies will cause failures
if the testsuite is modified.  Unfortunately, this is not caught by
distcheck because the testsuite is not rebuilt normally.  Therefore,
it is desirable for the dependencies to be picked up automatically
to reduce the possibility of omission.

We could check the M4 traces in a dist-hook, but then Autotest becomes
a dependency for "make dist", even when the testsuite is not modified.
Instead, distribute a record of all dependencies, and use that to
implement the dist-hook.