Make panel and panel.io lazy imports

[hoxbro]

Honestly, this should not be added to the 1.5 release, as it is too late.

But would not want to delete the changes either.

python -X importtime -c 'import panel' 2> tuna.log && tuna tuna.log
This PR:

Speed import PR

[codecov]

Codecov Report

Attention: Patch coverage is 62.12625% with 114 lines in your changes missing coverage. Please review.

Project coverage is 81.99%. Comparing base (d49dafc) to head (0a6d5dd).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/pane/__init__.py	40.54%	22 Missing ⚠️
panel/widgets/__init__.py	37.14%	22 Missing ⚠️
panel/io/__init__.py	48.78%	21 Missing ⚠️
panel/layout/__init__.py	42.85%	16 Missing ⚠️
panel/__init__.py	66.66%	10 Missing ⚠️
panel/config.py	73.91%	6 Missing ⚠️
panel/io/state.py	50.00%	4 Missing ⚠️
panel/util/__init__.py	73.33%	4 Missing ⚠️
panel/links.py	0.00%	2 Missing ⚠️
panel/pane/holoviews.py	66.66%	2 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7208      +/-   ##
==========================================
- Coverage   82.23%   81.99%   -0.25%     
==========================================
  Files         335      337       +2     
  Lines       49942    50364     +422     
==========================================
+ Hits        41071    41296     +225     
- Misses       8871     9068     +197     

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