Allow existing item with matching names in io.maybe_init_item()

[khaeru]

Follow-up for #289; required for iiasa/message_ix#321.

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PR checklist

• Tests added. The method is still 100% covered.
• Documentation added. N/A
• Release notes updated. N/A

[codecov]

Codecov Report

Merging #294 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   97.38%   97.38%           
=======================================
  Files          39       39           
  Lines        3979     3980    +1     
=======================================
+ Hits         3875     3876    +1     
  Misses        104      104           

Impacted Files	Coverage Δ
ixmp/backend/io.py	98.16% <100.00%> (ø)
ixmp/core.py	95.72% <0.00%> (+0.01%)	⬆️

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