feat: relative cross-references

Fixes mkdocstrings#27

This commit fixes:
 - the regular expression for checking valid python identifiers
 - update of line offset into doc string for error messages

src/mkdocstrings_handlers/python/crossref.py

@@ -75,7 +75,7 @@ def _re_named(name: str, exp: str, optional: bool = False) -> str:
 - 'current': an expression of the form '.'
 """
 
-_RE_ID = re.compile("[a-zA-Z][a-zA-Z_.]*")
+_RE_ID = re.compile("[a-zA-Z_][a-zA-Z0-9_.]*")
 """Regular expression that matches a qualified python identifier."""
 
 
@@ -221,7 +221,7 @@ def _error(self, msg: str) -> None:
             line = doc.lineno
             if line is not None:
                 # Add line offset to match in docstring
-                line + doc.value.count("\n", 0, self._cur_offset)
+                line += doc.value.count("\n", 0, self._cur_offset)
                 prefix += f"{line}:"
                 # It would be nice to add the column as well, but we cannot determine
                 # that without knowing how much the doc string was unindented.

tests/test_crossref.py

@@ -106,3 +106,5 @@ def test_substitute_relative_crossrefs(caplog):
     [bar][mod1.mod2.bar]
     """
     )  # noqa: WPS355
+
+    assert not caplog.records