Fix wrong refid in Doxygen's XML generator

Given the following Doxygen input:

```
/**
 * This struct must be used with f()
 */
struct t {
  int x;
}

/**
 * @param tx A struct t pointer
 */
void f(struct t *tx) {
  (void)tx;
}
```

The `f()` in struct t's comment generates a <ref> element with `refid`
equal to `some_prefix_compoundid_anchorid`, wheres the `id` in the
refered element ends up with an `id` equal to `some_prefix_anchorid`.
The anchorid here is just the compoundid prefix by "_1", so the `refid`
actually has this information duplicated.

This is a band-aid solution that tries to detect the issue in the ref
ids and adds the duplication, so that resulting links work.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

breathe/renderer/target.py

@@ -16,9 +16,26 @@ def __init__(self, project_info: ProjectInfo, document: nodes.document):
         self.project_info = project_info
         self.document = document
 
+    # XXX: when inside a group in Doxygen, if the XML generator finds a
+    # reference in a comment, it generates a new <ref> element where the refid
+    # gets both a compoundid and an anchorid, where: anchorid = "_1" + compoundid.
+    # In the group__*.xml files the ids that are linked to, only get the
+    # anchorid (or "_1" + compoundid!).
+    # This function tries to detect if the anchor is not present twice, and
+    # fixup the ref id so that the final symbol resolution works.
+    def _fixup_target(self, refid: str) -> str:
+        if refid:
+            parts = refid.rsplit("_", 1)
+            if len(parts) == 2 and parts[1].startswith("1"):
+                anchorid = parts[1][1:]
+                if anchorid not in parts[0]:
+                    return "_".join([parts[0], anchorid, parts[1]])
+        return refid
+
     def create_target(self, refid: str) -> List[Element]:
         """Creates a target node and registers it with the document and returns it in a list"""
 
+        refid = self._fixup_target(refid)
         target = nodes.target(ids=[refid], names=[refid])
         try:
             self.document.note_explicit_target(target)