rewrite check_files_md5 in pure Python

[bdrung]

GNU coreutils 9.11 and rust-coreutils 0.9 changed md5sum to quote the filenames in case they contain spaces or other special characters. Example:

$ echo first > "with spaces"
$ echo second > "with'quotes"
$ echo third > 'with"quotes'
$ echo fourth > "with \" and ' quotes"
$ md5sum "with spaces" "with'quotes" 'with"quotes' "with \" and ' quotes" > sums
$ md5sum -c sums
'with spaces': OK
"with'quotes": OK
'with"quotes': OK
'with " and '\'' quotes': OK

This new quoting could be supported by checking for single or double quotes and use shlex.split in these cases.

To make the code more robust do not rely on the md5sum command and just use pure Python for it.

Bug: https://launchpad.net/bugs/2161957

[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.47%. Comparing base (2eacb11) to head (7cc8df5).

Files with missing lines	Patch %	Lines
apport/packaging_impl/apt_dpkg.py	72.72%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #643      +/-   ##
==========================================
- Coverage   84.48%   84.47%   -0.02%     
==========================================
  Files         106      106              
  Lines       21018    20996      -22     
  Branches     3195     3187       -8     
==========================================
- Hits        17758    17736      -22     
- Misses       2785     2786       +1     
+ Partials      475      474       -1     

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