wic: Check for external modules

wic: Check for external modules

Since eight unique files import rpm, perform a check at the top level
for the existence of the rpm module print a sensible error message if it
is not. This may be able to be removed if some of the core rpm
dependencies are removed from the mic libs.

Also check for urlgrabber.

This avoids a bracktrace in the event the modules are not installed
which can be very off-putting to would-be users.

(From OE-Core rev: b11bfadba20c1f39a63e396e605a8316c2ed2a94)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>