[core] Add ProcessEnv class in new plugins module

Add a new module named `plugins` with a `ProcessEnv` class which
contains the paths describing the environment needed for the plugin's
(node's) process.

meshroom/core/plugins.py

@@ -0,0 +1,14 @@
+from pathlib import Path
+
+from meshroom.common import BaseObject
+
+class ProcessEnv(BaseObject):
+    """
+    Describes the environment required by a node's process.
+    """
+
+    def __init__(self, folder: str):
+        super().__init__()
+        self.binPaths: list = [Path(folder, "bin")]
+        self.libPaths: list = [Path(folder, "lib"), Path(folder, "lib64")]
+        self.pythonPathFolders: list = [Path(folder)] + self.binPaths