extensions: make `hg nonexistent` not crash with PyOxidizer

When running `hg nonexistent`, we try to look for extensions that
provide that command. We do that by looking for files in the
`hgext.__file__` directory. However, PyOxidizer doesn't provide a
`__file__`, so we crash when running with PyOxidizer.

We should be able to look for the command in built-in extensions, but
we seem to already have code for skipping the scan when running in a
frozen binary, so I just modified that code instead.

By the way, it also seems like we should be able to search for
extensions in the `hgext3rd` module, but we don't do that yet either
(before or after this patch).

Differential Revision: https://phab.mercurial-scm.org/D8750

mercurial/extensions.py

@@ -706,12 +706,17 b' def _disabledpaths():'
     '''find paths of disabled extensions. returns a dict of {name: path}'''
     import hgext
 
-    extpath = os.path.dirname(
-        os.path.abspath(pycompat.fsencode(hgext.__file__))
-    )
-    try:  # might not be a filesystem path
-        files = os.listdir(extpath)
-    except OSError:
+    # The hgext might not have a __file__ attribute (e.g. in PyOxidizer) and
+    # it might not be on a filesystem even if it does.
+    if util.safehasattr(hgext, '__file__'):
+        extpath = os.path.dirname(
+            os.path.abspath(pycompat.fsencode(hgext.__file__))
+        )
+        try:
+            files = os.listdir(extpath)
+        except OSError:
+            return {}
+    else:
         return {}
 
     exts = {}