Add Tox plugin to work around cyclic dependency on verifai (#251)

toxfile.py

@@ -0,0 +1,17 @@
+import tox.plugin
+
+
+@tox.plugin.impl
+def tox_on_install(tox_env, arguments, section, of_type):
+    # Add extra installation stage to install Scenic *before* its dependencies,
+    # so that if we install VerifAI we won't pull in a different version of
+    # Scenic that might have different dependencies. (This is currently
+    # necessary since Tox installs the dependencies and Scenic in 2 different
+    # invocations of pip. See https://github.com/tox-dev/tox/discussions/3273
+    # for a discussion.)
+    if section == "RunToxEnv" and of_type == "package":
+        assert len(arguments) == 1
+        package = arguments[0]
+        path = package.path
+        install_args = ["--force-reinstall", "--no-deps", str(path)]
+        tox_env.installer._execute_installer(install_args, "package_early")