Cross reference in separate files with rule name same as type name raises error

The error Use the rule type 'X' instead of the typed rule name 'X' for cross references. is raised when

1. The name of the referenced type and the name of its parse rule are the same,
2. The parse rule of the referenced type and the cross reference are placed in different files.

Using different name or placing them in one file can remove this error.

A minimal example:

// file debug.langium
grammar Debug

import './foo'

entry Model:
    (b+=B | a+=A)*;

// A infers A:
//     'A' a=ID;

B infers B:
    'B' b=[A:ID];

// file foo.langium
A infers A:
    'A' a=ID;

terminal ID: /[_a-zA-Z][\w_]*/;

Renaming the rule A or moving it into debug.langium both resolve the error.

[spoenemann]

It would be great if we can add a test for these two issues

See #768

[msujew]

Closed due to https://github.com/langium/langium/pull/768