Make version in `pyproject.toml` optional, or at least validate it *after* the plugins are run

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

Thanks for Poetry! I'm a big fan. 🎉

I wanted to be able to keep my package versions in __init__.py, and with the new alpha supporting plugins I was able to create a plugin to read the version from it dynamically: https://github.com/tiangolo/poetry-version-plugin

But currently, Poetry checks and verifies if the version is set in pyproject.toml, and if it is not set there, it terminates with an error.

That means that even though the plugin is taking care of setting the package version, a dummy "version" is required in pyproject.toml, even though it is not really used.

My feature request is to make the version field optional in pyproject.toml.

Or alternatively, check if the version is set in the Poetry object in memory after running the plugins. That way my plugin can set the version.

Motivation

If users try to use that plugin (or any other similar one), they would have to set a version just to trick Poetry into thinking the pyproject.toml has all the fields, while the version is not really used in the end. So, for the final result, it's an extra workaround step.

Start by tracing where Poetry validates the version in pyproject.toml and when plugins run; the issue names no files or tests. Compare making the field optional with validating the in-memory version after plugins, and consider the behavior for projects using dynamically supplied versions.