Param private name space: handle initialized

[maximlt]

Parameterized objects are going to have a private namespace starting from Param 2.0 (holoviz/param#766). This PR handles the initialized attribute that is moving to this namespace.

@philippjfr let me know when you make a dev release including this PR as it will unblock holoviz/param#766.

[codecov]

Codecov Report

Merging #5198 (25f3900) into main (f7f9ea9) will decrease coverage by 0.17%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main    #5198      +/-   ##
==========================================
- Coverage   83.82%   83.65%   -0.17%     
==========================================
  Files         274      274              
  Lines       39594    39600       +6     
==========================================
- Hits        33189    33127      -62     
- Misses       6405     6473      +68     

Flag	Coverage Δ
ui-tests	39.52% <87.50%> (-1.02%)	⬇️
unitexamples-tests	73.77% <87.50%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
panel/config.py	76.01% <87.50%> (-0.34%)	⬇️

... and 10 files with indirect coverage changes

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[philippjfr]

Looks good, thank you!