Keep all packed charms, if there are more than one (#115)

* Update plugin.py

* linting...

* Address 'referenced before assignment'

---------

Co-authored-by: Adam Dyess <adam.dyess@canonical.com>

Author: sed-i

pytest_operator/plugin.py

@@ -931,7 +931,7 @@ async def build_charm(
         verbosity: Optional[
             Literal["quiet", "brief", "verbose", "debug", "trace"]
         ] = None,
-    ) -> Path:
+    ) -> Optional[Path]:
         """Builds a single charm.
 
         This can handle charms using the older charms.reactive framework as
@@ -1006,9 +1006,15 @@ async def build_charm(
                 f"Failed to build charm {charm_path}:\n{stderr}\n{stdout}"
             )
 
-        charm_file_src = next(charm_abs.glob(f"{charm_name}*.charm"))
-        charm_file_dst = charms_dst_dir / charm_file_src.name
-        charm_file_src.rename(charm_file_dst)
+        # If charmcraft.yaml has multiple bases
+        # then multiple charms would be generated.
+        charm_file_dst = None
+        for charm_file_src in charm_abs.glob(f"{charm_name}*.charm"):
+            charm_file_dst = charms_dst_dir / charm_file_src.name
+            charm_file_src.rename(charm_file_dst)
+
+        # Even though we may have multiple *.charm file,
+        # for backwards compatibility we can - only return one.
         return charm_file_dst
 
     async def build_charms(self, *charm_paths) -> Mapping[str, Path]: