fix: use find_spec for pip plugins

For pip plugins installed in editable mode, via `pip install -e ...`,
buildstream failed to resolve the actual location of source files of a
plugin.
This is caused because `importlib.metadata` does not honor editable
packages with `pth` files.

This patch updates `pluginoriginpip.py` to use `find_spec` to find
files. This works for editable packages.

Author: kotborealis

src/buildstream/_pluginfactory/pluginoriginpip.py

@@ -11,7 +11,7 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 #
-import os
+from pathlib import Path
 
 from .._exceptions import PluginError
 
@@ -34,6 +34,7 @@ def get_plugin_paths(self, kind, plugin_type):
 
         from packaging.requirements import Requirement, InvalidRequirement
         from importlib.metadata import distribution, PackageNotFoundError
+        from importlib.util import find_spec
 
         # Sources and elements are looked up in separate
         # entrypoint groups from the same package.
@@ -86,17 +87,19 @@ def get_plugin_paths(self, kind, plugin_type):
                 reason="plugin-not-found",
             )
 
-        location = dist.locate_file(entrypoint.module.replace(".", os.sep) + ".py")
-        defaults = dist.locate_file(entrypoint.module.replace(".", os.sep) + ".yaml")
+        location = Path(find_spec(entrypoint.module).origin)
+        defaults = location.with_suffix(".yaml")
 
         if not defaults.exists():
             # The plugin didn't have an accompanying YAML file
             defaults = None
 
+        directory = str(location.parent)
+
         return (
-            os.path.dirname(location),
+            directory,
             str(defaults),
-            "python package '{}' version {} at: {}".format(dist.name, dist.version, dist.locate_file("")),
+            "python package '{}' version {} at: {}".format(dist.name, dist.version, directory),
         )
 
     def load_config(self, origin_node):