Rework plugin initialisation points

In general this patch pushes plugin initialisation points to earlier in
the pipeline. As plugins can modify the `HscEnv`, it's imperative that
the plugins are initialised as soon as possible and used thereafter.

For example, there are some new tests which modify hsc_logger and other
hooks which failed to fire before (and now do)

One consequence of this change is that the error for specifying the
usage of a HPT plugin from the command line has changed, because it's
now attempted to be loaded at initialisation rather than causing a
cyclic module import.

Closes #21279

Co-authored-by: Matthew Pickering <matthewtpickering@gmail.com>