Tidy up mx-bluesky parameter packages

The `mx_bluesky.hyperion.parameters` package is full of unrelated things:

* `cli.py` has nothing to do with the parameter model
* `gridscan.py` has device-level parameter builders that are unrelated to the mx-bluesky parameter model
* `HyperionGridDetectThenXRayCentreComposite` is not part of the parameter model. Arguably it should be defined with the experiment plans that use it, or perhaps we should create a new composites package to contain the composite types used by the various plans.

## Acceptance Criteria
- Specific criteria that will be used to judge if the issue is fixed

Start by inspecting the mx_bluesky.hyperion.parameters package, especially cli.py, gridscan.py, and HyperionGridDetectThenXRayCentreComposite, then trace their usages to understand appropriate destinations. Done means unrelated CLI, device-level builders, and composite code no longer make the parameter package contain unrelated responsibilities, with the chosen package structure documented by the resulting imports and tests.

Written by the indexing model from the issue text.