sort properly Go imports

[dmitris]

Summary

As in the previously merged #4061 and #4062, sort the Go imports to be in the alphabetical order (mostly the "third-party" = non-stdlib imports though pkg/cosign/errors_test.go had the stdlib imports in a wrong order).
As mentioned in #4062,

I propose to keep all the third-party (non-stdlib) imports in just one block to make it easier for the IDEs to properly sort those imports on addition or deletion.

I didn't change the existing extra blank lines where they don't violate the alphabetical sorted order of the imports.

Release Note

NONE

Documentation

n/a

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.76%. Comparing base (2ef6022) to head (63ec684).
Report is 315 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4071      +/-   ##
==========================================
- Coverage   40.10%   36.76%   -3.35%     
==========================================
  Files         155      210      +55     
  Lines       10044    13386    +3342     
==========================================
+ Hits         4028     4921     +893     
- Misses       5530     7847    +2317     
- Partials      486      618     +132     

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