Bring back async_cmd_check to check for parallel extension install completion and fix dry-run mode with parallel extensions - #5223

We currently have this documentation

and check whether the command has completed via the async_cmd_check method.

It is used by rpackage which calls into the super method: https://github.com/easybuilders/easybuild-easyblocks/blob/e53ee56b0ddce722fb4b37371096b0eabae6cf6f/easybuild/easyblocks/generic/rpackage.py#L335

However it was removed by 36febd8 so the rpackage would crash.

It's true that async_cmd_check is still defined by the RPackage easyblock, but it serves no purpose anymore, I don't see how it could lead to a crash?

I don't see why we would reinstate async_cmd_check, which was removed when we switched to using run_shell_cmd for running shell commands asynchronously in:

• add support for running shell commands asynchronously with run_shell_cmd #4444

So instead, I think we should:

• remove the async_cmd_check method from RPackage easyblock;
• update our to correctly reflect what's required in order to be able to install extensions in parallel;

I don't see why we would reinstate async_cmd_check, which was removed when we switched to using run_shell_cmd for running shell commands asynchronously in:

The rpackage easyblock shows that this is a useful hook to check that the async command actually succeeded that simply checking the completion marker of the task doesn't allow: https://github.com/easybuilders/easybuild-easyblocks/blob/develop/easybuild/easyblocks/generic/rpackage.py#L335-L337

The base implementation of async_cmd_check does the simple: if not self.async_cmd_task.done():
But it also stores the results for use by the custom easyblocks, e.g. in the hook.

So I don't see this as a replacement but an enhancement and think we should keep it.
We could remove self.async_cmd_output as I guess custom easyblocks could get that via self.async_cmd_task.result() although it seems convenient