replace _ExternalRef logic with persistent_id

[mike0sv]

also
closes #364

[codecov]

Codecov Report

Merging #365 (00f411e) into main (95d521b) will increase coverage by 5.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
+ Coverage   84.37%   89.49%   +5.12%     
==========================================
  Files          81       81              
  Lines        6297     6275      -22     
==========================================
+ Hits         5313     5616     +303     
+ Misses        984      659     -325     

Impacted Files	Coverage Δ
mlem/core/objects.py	91.42% <ø> (+4.93%)	⬆️
mlem/contrib/callable.py	98.26% <100.00%> (-0.19%)	⬇️
mlem/core/metadata.py	95.91% <100.00%> (+2.93%)	⬆️
mlem/contrib/dvc.py	60.29% <0.00%> (-0.90%)	⬇️
mlem/utils/root.py	91.66% <0.00%> (+0.23%)	⬆️
mlem/core/requirements.py	95.05% <0.00%> (+0.38%)	⬆️
mlem/core/meta_io.py	94.11% <0.00%> (+0.53%)	⬆️
mlem/utils/module.py	90.11% <0.00%> (+0.76%)	⬆️
mlem/api/commands.py	88.88% <0.00%> (+2.22%)	⬆️
mlem/contrib/sklearn.py	81.25% <0.00%> (+3.12%)	⬆️
... and 11 more

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