Allow using *relative imports* to cross-reference inside the project

[leandro-lucarella-frequenz]

Is your feature request related to a problem? Please describe.

When writing cross-references inside the project, links become a bit too verbose. For example, if we have:

# module some.deep.nested.module_a
class A:
    """A class.

    We reference [B][some.deep.nested.module_b.B].
    """

class AA:
    """AA class.

    We reference [A][some.deep.nested.module_a.A].
    """

# module some.deep.nested.module_b
class B:
    """B class.

    We reference [A][some.deep.nested.module_a.A].
    """

Here references can get really difficult to write. Another alternative would be to write [some.deep.nested.module_a.A][], which saves a few chars (depending on the name of the leaf symbol), but then the produced documentation becomes too verbose if we are already at the context of a some.deep.nested package.

Describe the solution you'd like

Allow writing local cross-references using relative imports and local symbols. In the example above:

# module some.deep.nested.module_a
class A:
    """A class.

    We reference [B][.module_b.B].
    """

class AA:
    """AA class.

    We reference [A][].
    """

# module some.deep.nested.module_b
class B:
    """B class.

    We reference [.module_a.A][].
    """

Describe alternatives you've considered

Just using the fully qualified symbol as stated above.

Additional context

None really, just that I know it would be hard to implement :)

[analog-cbarber]

See mkdocstrings/python#27 and the corresponding PR mkdocstrings/python#28

That just solves the problem for python but I expect most users are using the python handler.