Cleanup: use plural names for Main lists

Convention was not to but after discussion on 918941483f we agree its
best to change the convention.

Names now mostly follow RNA.

Some exceptions:

- Use 'nodetrees' instead of 'nodegroups'
  since the struct is called NodeTree.
- Use 'gpencils' instead of 'grease_pencil'
  since 'gpencil' is a common abbreviation in the C code.

Other exceptions:

- Leave 'wm' as it's a list of one.
- Leave 'ipo' as is for versioning.

source/blender/python/intern/bpy_rna.c

@@ -2343,7 +2343,7 @@ static int pyrna_prop_collection_subscript_str_lib_pair_ptr(
 		else if (PyUnicode_Check(keylib)) {
 			Main *bmain = self->ptr.data;
 			const char *keylib_str = _PyUnicode_AsString(keylib);
-			lib = BLI_findstring(&bmain->library, keylib_str, offsetof(Library, name));
+			lib = BLI_findstring(&bmain->libraries, keylib_str, offsetof(Library, name));
 			if (lib == NULL) {
 				if (err_not_found) {
 					PyErr_Format(PyExc_KeyError,