refactor(sdk): combine _safe_mkdirs with mkdir_exist_ok

[moredatarequired]

Fixes WB-11844

These aren't exactly identical helper functions, but they're used identically (in particular, there are no callers that make use of the returned boolean value from mkdir_exist_ok). This combines them, using the _safe_mkdirs code and location (lib.filesystem) but using the name of the more commonly used mkdir_exist_ok.

[codecov]

Codecov Report

Merging #4650 (b660806) into main (77f2c8c) will increase coverage by 0.01%.
The diff coverage is 87.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4650      +/-   ##
==========================================
+ Coverage   83.23%   83.25%   +0.01%     
==========================================
  Files         279      279              
  Lines       34927    34927              
==========================================
+ Hits        29071    29077       +6     
+ Misses       5856     5850       -6     

Flag	Coverage Δ
functest	55.94% <70.90%> (+0.02%)	⬆️
unittest	73.81% <85.45%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
wandb/old/core.py	56.92% <0.00%> (-0.89%)	⬇️
wandb/jupyter.py	56.06% <50.00%> (+0.16%)	⬆️
wandb/sdk/data_types/video.py	78.07% <50.00%> (+0.19%)	⬆️
wandb/sdk/wandb_artifacts.py	83.71% <66.66%> (+0.01%)	⬆️
wandb/cli/cli.py	69.33% <75.00%> (+0.02%)	⬆️
wandb/sdk/lib/filesystem.py	95.91% <80.00%> (+3.19%)	⬆️
wandb/apis/public.py	81.96% <100.00%> (ø)
wandb/data_types.py	82.88% <100.00%> (+0.01%)	⬆️
wandb/filesync/step_checksum.py	92.53% <100.00%> (+0.11%)	⬆️
wandb/old/settings.py	92.50% <100.00%> (+0.09%)	⬆️
... and 18 more