[WIP] Improve custom importer to pass parent import context

Needs some more work and more tests to make sure
it does not introduce any memory leaks!

One example would be a custom importer that loads the
content from the web. If this includes another import,
it should be loaded relative to the parent file. To do
this, the custom importer needs to know the context of
the parent import. Which is what this commit will fix!