Subclassing default rezplugins instead of replacing

Hi all,

We sometimes need to override official rezplugins for some reasons and keep the default name (local build_process, ...) to avoid specifying a different plugins in each command call.
Since 2.40.3 with #677 feature, we are able to replace completely a default plugin by our custom one and this is really great.

But a complete replace could be hard to maintain with the HEAD of official rezplugins and a subclassing should be better. It's also useful to give info only about what have been modified from the default plugin.

So a custom build_process/local.py should only contain:

```python
from rezplugins.build_process.local import LocalBuildProcess

class MyCustomLocalBuildProcess(LocalBuildProcess):
def release(self, release_message=None, variants=None):
"""MyCustom LocalBuildProcess release override."""
self._print_header("Custom releasing %s..." % self.package.qualified_name)

def register_plugin():
return MyCustomLocalBuildProcess
```

instead of a full copy (~300 lines) with a similar modification than above.

Currently this is not possible to import the official `local` module when we are using the custom `local` plugin, the plugin_manager skips import of similar plugins (to avoid reload ?).

What do you think about this possible way to do ?

Jordi

Start with the plugin_manager behavior that skips importing a similarly named plugin, then compare it with the custom build_process/local.py and official rezplugins.build_process.local example in the issue. Done means a custom plugin can subclass the official LocalBuildProcess while retaining the default plugin name and replacement behavior; check existing plugin-loading tests if present.

Written by the indexing model from the issue text.