Check [tool.poetry.requires-plugins] on non-`install` commands

Issue Kind

Change in current behaviour

Description

Currently, the requirements in [tool.poetry.requires-plugins] are only checked (and ensured) when poetry install is run. I propose that poetry will check them on every non-self usage (poetry update, poetry add, etc.), and will print a warning if the plugins aren't installed.

These commands will also have a new flag --no-required-plugin that will skip this check (as a bonus, we could also add this flag to poetry install, which will allow users to skip both the checking and installation of required plugins)

I'm willing to make a PR for this if the proposal is accepted.

Impact

At our organization, we'd like to make use of [tool.poetry.requires-plugins] in our projects to enforce the use of our in-house plugin, but it's currently problematic for two reasons:

• The requirement is only checked during poetry install, but the plugin is only actually needed for lock/add/update commands. This means that developers might clone a repo, and run poetry update, without having the plugin installed.
• Installing the required plugins is, AFAICT, mandatory when you use poetry install. This means that users that don't necessarily need the plugin (CI pipelines, docker builds), must still install it.

The proposed change will address both these issues.

Workarounds

Our current workaround is to simply not use [tool.poetry.requires-plugins], and to ensure all our devs install the required plugins.

Start by tracing how Poetry handles required plugins for install, then compare the command entry points for update, add, and other non-self commands. Define which commands check or install plugins, how the --no-required-plugin flag applies, and when warnings are emitted. Done means the proposed behavior is covered consistently without affecting self commands.