import-checker: don't treat modules as relative one if not found
The previous patch ensures all module names are recorded in `imports`
as absolute names, so we no longer need to treat modules as ones
imported relatively from the target source if they appear to not be
from the stdlib.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown