Issue 10327 - More informative error message on missing package.d file

http://d.puremagic.com/issues/show_bug.cgi?id=10327

Unfortunately I can't build a proper test-case because the compiler spits out import paths in the error message and these are machine-specific (e.g. imports=C:\dmd ..).

Note that the recently introduced `isPkgMod` field is unset at this point, so comparing strings seems like the only way to tell whether we're trying to import `package.d`.