Fixed non-native module importing.

Fixed non-native module importing.

Previously, string from "import statement" was used as a key in a hash
to keep track of already loaded modules. This works fine for built-in
modules. It can cause an issue when different modules from different
paths but with identical names are loaded. This patch avoids the issue
by using a full path to a file as a key.

This closes #282 issue on GitHub.