Since the recent refactoring, it's been reported that some USB-audio
devices (typically webcams) are no longer detected properly by
PulseAudio.  The debug session revealed that it's failing at probing
by PA to try the sample rate 44.1kHz while the device has discrete
sample rates other than 44.1kHz.  But the puzzle was that arecord
works as is, and some other devices with the discrete rates work,
either.

After all, this turned out to be the lack of the dependencies in a few
hw constraint rules: snd_pcm_hw_rule_add() has the (variable)
arguments specifying the dependent parameters, and some functions
didn't set the target parameter itself as the dependencies.  This
resulted in an invalid parameter that could be generated only in a
certain call pattern.  This bug itself has been present in the code,
but it didn't trigger errors just because the rules were casually
avoiding such a corner case.  After the recent refactoring and
cleanup, however, the hw constra...