Fatal error calling db_query() when module is specified in an install profile

I kept hitting an error during installation when I was using an install profile that used Path Cache:

PHP Fatal error:  Call to undefined function db_result() in /Users/amorton/Sites/d6-dev/sites/all/modules/contrib/pathcache/pathcache.install on line 16

Eventually I traced it to the install time hook_requirements() checks. Path Cache was assuming that the database driver had already been included which in this case it had not. Since this is only information at this point I think we only check at run time.