Moved panes and widgets into subpackage

[philippjfr]

No description provided.

[codecov-io]

Codecov Report

Merging #283 into master will increase coverage by 0.13%.
The diff coverage is 90.19%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   88.11%   88.25%   +0.13%     
==========================================
  Files          44       58      +14     
  Lines        5645     5712      +67     
==========================================
+ Hits         4974     5041      +67     
  Misses        671      671

Impacted Files	Coverage Δ
panel/tests/test_vega.py	96.61% <100%> (ø)	⬆️
panel/pipeline.py	80.68% <100%> (-0.14%)	⬇️
panel/models/__init__.py	100% <100%> (ø)
panel/pane/holoviews.py	82.35% <100%> (ø)
panel/widgets/base.py	100% <100%> (ø)
panel/tests/test_plotly.py	97.59% <100%> (ø)	⬆️
panel/interact.py	73.78% <100%> (+0.09%)	⬆️
panel/param.py	89.65% <100%> (+0.02%)	⬆️
panel/widgets/__init__.py	100% <100%> (ø)
panel/pane/vega.py	81.53% <100%> (ø)
... and 35 more

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