Add globus_cli_rmtree via toxfile plugin

This imitates our prior art in globus-sdk to move
`allowlist_externals = rm` usage into a plugin which can take care of
this for us. It also sets up `toxfile.py` for us in case we want
to put any other customizations in there.

tox.ini

@@ -78,8 +78,7 @@ skip_install = true
 deps =
     build
     twine
-allowlist_externals = rm
-commands_pre = rm -rf dist/
+globus_cli_rmtree = dist
 # check that twine validating package data works
 commands =
     python -m build

toxfile.py

@@ -0,0 +1,46 @@
+"""
+This is a very small 'tox' plugin.
+'toxfile.py' is a special name for auto-loading a plugin without defining package
+metadata.
+
+For full doc, see: https://tox.wiki/en/latest/plugins.html
+
+Methods decorated below with `tox.plugin.impl` are hook implementations.
+We only implement hooks which we need.
+"""
+
+from __future__ import annotations
+
+import logging
+import pathlib
+import shutil
+import typing as t
+
+from tox.plugin import impl
+
+if t.TYPE_CHECKING:
+    from tox.config.sets import EnvConfigSet
+    from tox.session.state import State
+    from tox.tox_env.api import ToxEnv
+
+log = logging.getLogger(__name__)
+
+
+@impl
+def tox_add_env_config(env_conf: EnvConfigSet, state: State) -> None:
+    env_conf.add_config(
+        keys=["globus_cli_rmtree"],
+        of_type=list[str],
+        default=[],
+        desc="A dir tree to remove before running the environment commands",
+    )
+
+
+@impl
+def tox_before_run_commands(tox_env: ToxEnv) -> None:
+    cli_rmtree = tox_env.conf.load("globus_cli_rmtree")
+    for name in cli_rmtree:
+        path = pathlib.Path(name)
+        if path.exists():
+            log.warning(f"globus_cli_rmtree: {path}")
+            shutil.rmtree(path)