Functions can also contain imports

[tristanlatr]

Here's a test that is currently failing:

@pytest.mark.xfail
@systemcls_param
def test_links_function_docstring_imports_in_body(systemcls: Type[model.System]) -> None:
    
    mod = '''
    def expandName(name: str) -> str:
        """
        See L{names.expandName}
        """
        from pydoctor import names
        return names.expandName(name)
    '''
    
    _impl = '''
    def expandName(name: str) -> str:
        """Docs"""
        ...
    '''
    
    system = systemcls()
    builder = system.systemBuilder(system)
    builder.addModuleString('', modname='pydoctor', is_package=True)
    builder.addModuleString(mod, modname='pydoctor.model')
    builder.addModuleString(_impl, modname='pydoctor.names')
    builder.buildModules()

    fn = system.allobjects['pydoctor.model.expandName']

    assert fn.expandName('names.expandName') == 'pydoctor.names.expandName'

I believe we need the distinction between CanContainImport and CanContainDocumentable.