Move loading and combine code into their own submodules.

[pp-mo]

Replaces #6199
This is supposedly a "pure refactor"
Except for some rewrites of docstrings, and changes to tests where they imported some (non-public) routines which have moved

Effectively the following things happened ..

• iris.LoadPolicy has been renamed CombineOptions -- but LoadPolicy still exists as an alternative name
  • I'm intending to go on to extend the features here, as explained in Provide "combine_cubes" function to integrate new merge/concatenate/load facilities #6302
  • I'm still not totally sure whether these two will remain identical. For now they do.
• the following definitions have moved from iris.__init__ and are now defined in iris.loading
  (but still publicly declared in the iris module) :
  LoadPolicy, LOAD_POLICY, load, load_cube, load_cubes and load_raw
  -- though LoadPolicy is now "really" CombinePolicy and the definition is actually in iris._combine
• some private routines also moved from iris/__init__ to iris.loading :
  _generate_cubes, _load_collection
• the following definitions have likewise moved from iris.cube to iris.loading (though these ones aren't public) :
  _CubeFilter, _CubeFilterCollection
• the following definitions moved from iris.__init__ to iris._combine :
  _combine_cubes, _combine_load_cubes, and CombineCubes (which before was LoadPolicy)

[codecov]

Codecov Report

Attention: Patch coverage is 95.05495% with 9 lines in your changes missing coverage. Please review.

Project coverage is 89.88%. Comparing base (ca863a0) to head (944b1b5).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/_combine.py	92.95%	4 Missing and 1 partial ⚠️
lib/iris/loading.py	96.22%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6321   +/-   ##
=======================================
  Coverage   89.88%   89.88%           
=======================================
  Files          88       90    +2     
  Lines       23430    23447   +17     
  Branches     4361     4361           
=======================================
+ Hits        21059    21076   +17     
  Misses       1644     1644           
  Partials      727      727           

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[stephenworsley]

Looks good, much neater this way.

[pp-mo]

Thanks @stephenworsley !