Pass --explicit-package-bases to mypy by default (DataDog#23742)

* Add --explicit-package-bases to default mypy_args

Always prepend --explicit-package-bases to the mypy command built by the
datadog-checks Hatch environment collector. The flag is structurally
required by every integration's datadog_checks/<name>/ namespace package
layout, so integrations that enable check-types = true no longer need to
repeat it in their own hatch.toml. It is also a no-op on the few
integrations that ship a datadog_checks/__init__.py.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Add changelog

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: luisorofino

ddev/changelog.d/23742.added

@@ -0,0 +1 @@
+Add --explicit-package-bases to default mypy_args

ddev/src/ddev/plugin/external/hatch/environment_collector.py

@@ -63,7 +63,9 @@ def on_config(self, name: str, on_true: Any, on_false: Any) -> Any:
 
     @cached_property
     def mypy_args(self):
-        return self.config.get('mypy-args', []) + ['--install-types', '--non-interactive']
+        return (
+            ['--explicit-package-bases'] + self.config.get('mypy-args', []) + ['--install-types', '--non-interactive']
+        )
 
     @cached_property
     def mypy_files(self):