importer: Split out the code that evaluates the script

from the one that defines it on the import object itself. We'll soon
use this to implement an alternate require(); syntax that aligns with
CommonJS.
importer: Remove finish_import

We already check for pending exceptions at the import_file level,
so we don't need to do it again. Also, the log message mentions
native modules, which can be confusing.
importer: Replace load_module_init implementation with import_file
Support using GResource as a base search path for JS files

This commit changes the way files are imported inside GJS to use GIO,
transparently adding support for using a resource:/// search path to
lookup JS files in loaded GResources.

https://bugzilla.gnome.org/show_bug.cgi?id=703440
Rework the native module importing scheme

Make modules always supply their own module object when imported, and
simply define the properties that we want to on it afterwards. This
simplifies the structure of how native modules work and allows us to
remove a significant amount of support code for it.