Support --tag and --tag-only with nop publisher

[imjasonh]

Supercedes #689

cc @adambkaplan

With this change, if you use --push=false and -t, the output ref includes the tag. If you include --tag-only the resulting output ref will only include the tag, and not the digest. I'm not sure what purpose that would be useful for, but it's consistent with pushing cases at least.

[codecov-commenter]

Codecov Report

Merging #797 (d325733) into main (2c58e4a) will decrease coverage by 0.05%.
The diff coverage is 47.05%.

@@            Coverage Diff             @@
##             main     #797      +/-   ##
==========================================
- Coverage   51.45%   51.40%   -0.06%     
==========================================
  Files          44       44              
  Lines        3339     3354      +15     
==========================================
+ Hits         1718     1724       +6     
- Misses       1401     1408       +7     
- Partials      220      222       +2     

Impacted Files	Coverage Δ
pkg/commands/resolver.go	34.32% <47.05%> (+0.29%)	⬆️

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[adambkaplan]

@imjasonh thanks for getting this over the finish line!

[imjasonh]

@imjasonh thanks for getting this over the finish line!

Thanks for noticing the issue and for your very helpful PR!