Remove required `__init__.r` files in supermodules

FIx #16 modulo the need to check for module init files more smartly
(which needs fixing anyway with relative import paths), and the
amendment of test cases.
Add test for #18 and #24
Make imports with leading dot always relative

Fix #18.
Make imports absolute by default

Reorder the import search path to make imports non-local by default.
This fixes #24.
Don’t change directory to imported source file

Remove `chdir` argument to `source` in `import`, which is logically not
necessary any more, and was never desired.
Remove unnecessary setup in unit tests
Simplify sourcing modules

No need for nested `local` call.
Merge branch 'simplify-source' into issue-18
Adjust test import paths
Simplify logic
Fix bug in finding the calling module’s path

Fix a bug in the previous logic whereby a module calling `import` was
not found internally when looking for the module’s path, and the current
working directory was used instead.

Resolves #18 and closes #24.
Merge branch 'issue-18' into develop