Use imports to collect (scoped) type information

[lagru]

Closes #20.

Adds support for collecting additional type information from existing imports in the parsed package. Also does quite a bit of internal refactoring and maintenance.

E.g. with this

import dogs
from calendar import January as Jan, Day

def last_day(m, d):
    """
    Parameters
    ----------
    m : Jan
    d : dogs.Bernese

    Returns
    -------
    day : Day
    """

will work out of the box. Jan or Day will be picked up as implicitly available symbols in the given module. dogs will be collected as an import prefix in the module. They won't be available in other modules that don't feature these import statements.

We could also make these available to be used in other modules but for now I think this more conservative approach should be a good trade-off. It should save users from having to specify a large number of types manually. And weird import choices or naming conflicts are guarded by the scoping.

However, calendar.January will be collected as a global type and can be used in other modules.

This makes it more apparent that a full reference on docstubs type inference would be good to have in the documentation.

Release note

Use import statements in the parsed package to collect additional type
information. This makes imported types available in the same module
scope to be used in docstrings. Some of the imports can be used package
wide even if not imported there. E.g. `from pathlib import Path` will
allow using "Path" inside the modules scope. Outside of that module, 
you must use the full `pathlib.Path` to reference that type.
{label="enhancement"}

Type nicknames specified in the configuration are now resolved recursively.
That means one nickname can point to another nickname.
{label="enhancement"}