Add a module-level docstring.

This also makes the 'import' statements uniform.
Use 'project' for finding occurrences.

'pycore's functions aren't used when finding occurrences, so it's better
to just use 'project' directly.
Move 'get_string_module' and 'get_string_scope' into libutils.

These methods didn't use anything within the 'pycore' object except to
pass that in as a parameter. It's better to have people call a library
function instead and use the 'project' object without having to access
'pycore' directly.

No functionality change intended.
Pass in the 'project' and not the 'pycore'.