Save extra step of enabling installed plugins

Feature

mypy should enable installed plugins by default or limit the amount of toil required to enable plugins.

Pitch

Currently, enabling a plugin requires a multi-step process:

1. Install the plugin.
2. Create a config file.
3. Enable the plugin.

I imagine instead a world where:

Plugins are enabled by default

In this world, the act of installing a plugin would register a hook or entry point allowing the plugin(s) to be activated by default. In this world, the three-step process becomes a one-step process (install the plugin). One could disable the plugin by uninstalling (or not installing it in the first place) or perhaps through an additional configuration option (that presumably very few users would need).

Plugins can be enabled through the command-line

In this world, the three-step process becomes a two-step process, installing the plugin and enabling it during execution. This approach would limit the need to create mypy.ini files for small repro scripts or other impermanent actions. It would also make it more explicit what plugins are being used for a given invocation. Otherwise, to create a mypy repro that uses a plugin, a user is required to create a file so must communicate the creation of that file (and likely also clean it up after).

The issue names no files, tests, or entry points; start by tracing mypy's current plugin installation and configuration flow described in the three-step process. Compare the proposed automatic and command-line activation approaches, then clarify the desired behavior and configuration semantics. Done means the agreed approach reduces the setup required to use an installed plugin.