Add quotes around use of $env{MODALIAS} in rules.d/80-drivers.rules. The
modalias can contain whitespace, for example when it is dynamically generated
using device or vendor IDs.
This change is only about the source tree. We have tmpfiles.d/, modprobe.d/,
sysctl.d/, and sysusers.d/, but for historical reasons, rules/ didn't fit this
pattern. We also *install* it as rules.d/. Let's rename to be consistent.