cli/command: move prompt utilities to separate package

[thaJeztah]

See individual commits for details; may be doing some follow-ups after this.

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 67.94872% with 25 lines in your changes missing coverage. Please review.

Project coverage is 59.12%. Comparing base (8633197) to head (b74b7b3).
Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5952      +/-   ##
==========================================
- Coverage   59.13%   59.12%   -0.02%     
==========================================
  Files         355      356       +1     
  Lines       29744    29750       +6     
==========================================
  Hits        17590    17590              
- Misses      11179    11186       +7     
+ Partials      975      974       -1     

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[crazy-max]

could we have it in pkg so we can use it in buildx repo (https://github.com/docker/buildx/blob/646df6d4a06848c675abf110dd51c2623b517590/commands/util.go#L15-L57)?

[thaJeztah]

Ah, right, but that would probably not work currently? i.e., that fork was because of docker/buildx#2359 (comment), or was that part of it reverted?

My thinking of keeping it "internal" for now at least, was because there may still be some work to do to get a stable "API" for these. There's some functions that currently depend on command.Stream (or equivalent), and ideally they'd be more generic (I'm a bit on the fence on the Stream wrappers; they're useful, but perhaps we can do without so that we can pass a regular os.StdIn / os.StdErr etc).

[thaJeztah]

Rebase was for conflict in the imports; CI is happy again, so bringing this one in