[core] plugins: Add a Plugin class that contains a set of nodes

A `Plugin` object contains a collection of nodes, represented as
`NodePlugin` objects. This commit adds the implementation of the `Plugin`
class with the methods to add and remove `NodePlugins`, as well as an
empty class for the `NodePlugin` objects themselves.

meshroom/core/plugins.py

@@ -1,6 +1,11 @@
+from __future__ import annotations
+
+import logging
+
 from pathlib import Path
 
 from meshroom.common import BaseObject
+from meshroom.core import desc
 
 class ProcessEnv(BaseObject):
     """
@@ -12,3 +17,73 @@ class ProcessEnv(BaseObject):
         self.binPaths: list = [Path(folder, "bin")]
         self.libPaths: list = [Path(folder, "lib"), Path(folder, "lib64")]
         self.pythonPathFolders: list = [Path(folder)] + self.binPaths
+
+
+class Plugin(BaseObject):
+    """
+    A collection of node plugins.
+
+    Members:
+        name: the name of the plugin (e.g. name of the Python module containing the node plugins)
+        path: the absolute path of the plugin
+        _nodePlugins: dictionary mapping the name of a node plugin to its corresponding
+                      NodePlugin object
+        processEnv: the environment required for the nodes' processes to be correctly executed
+    """
+
+    def __init__(self, name: str, path: str):
+        super().__init__()
+
+        self._name: str = name
+        self._path: str = path
+
+        self._nodePlugins: dict[str: NodePlugin] = {}
+        self._processEnv: ProcessEnv = ProcessEnv(path)
+
+    @property
+    def name(self):
+        """ Return the name of the plugin. """
+        return self._name
+
+    @property
+    def path(self):
+        """ Return the absolute path of the plugin. """
+        return self._path
+
+    @property
+    def processEnv(self):
+        """ Return the environment required to successfully execute processes. """
+        return self._processEnv
+
+    def addNodePlugin(self, nodePlugin: NodePlugin):
+        """
+        Add a node plugin to the current plugin object and assign it as its containing plugin.
+        The node plugin is added to the dictionary of node plugins with the name of the node
+        descriptor as its key.
+
+        Args:
+            nodePlugin: the NodePlugin object to add to the Plugin.
+        """
+        self._nodePlugins[nodePlugin.nodeDescriptor.__name__] = nodePlugin
+        nodePlugin.plugin = self
+
+    def removeNodePlugin(self, name: str):
+        """
+        Remove a node plugin from the current plugin object and delete any container relationship.
+
+        Args:
+            name: the name of the NodePlugin to remove.
+        """
+        if name in self._nodePlugins:
+            self._nodePlugins[name].plugin = None
+            del self._nodePlugins[name]
+        else:
+            logging.warning(f"Node plugin {name} is not part of the plugin {self.name}.")
+
+
+
+class NodePlugin(BaseObject):
+    """
+    """
+    def __init__(self, nodeDesc: desc.Node, plugin: Plugin = None):
+        super().__init__()