Ensure hvplot.<ext> hooks are run on every import

[philippjfr]

The hvplot.<ext> import mechanism is a convenient way to allow users to have to avoid running the holoviews/panel extensions. However since imports are cached only the first import actually embeds the extension JS code, meaning that if you re-run the cell(s) containing import hvplot.pandas (or some other integration) then the JS will no longer be available and on subsequent reloads/re-runs of the notebook plots may not appear. This probably accounts for most issues encountered and described in https://github.com/holoviz/hvplot/issues/1232.

Here we add an IPython hook which simply deletes the modules before every cell execution. This is a big hammer but the best @hoxbro and I could find and at least it is restricted to IPython environments.

[hoxbro]

We need to update the logic to only register hvplot extensions once, have updated it for some but we should update it for all.

[maximlt]

We need to look more into how this is going to affect Polars' users.

[maximlt]

@hoxbro can you please review?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.95833% with 25 lines in your changes missing coverage. Please review.

Project coverage is 88.73%. Comparing base (6c96c7e) to head (763392d). Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
hvplot/cudf.py	0.00%	13 Missing :warning:
hvplot/intake.py	0.00%	7 Missing :warning:
hvplot/__init__.py	64.28%	5 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1359      +/-   ##
==========================================
+ Coverage   87.39%   88.73%   +1.34%     
==========================================
  Files          50       51       +1     
  Lines        7490     7592     +102     
==========================================
+ Hits         6546     6737     +191     
+ Misses        944      855      -89     

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[michaelaye]

oh my word, this explains so much, thanks for hunting it down...