opts: remove uses pkg/errors, and move swarm-specific opts to a separate package

[thaJeztah]

opts: use stdlib errors and touch-up some errors

• remove uses of github.com/pkg/errors
• slight improvement on handling parsing errors
• add some test-cases

opts: move swarm-specific options to a separate package

This prevents users of the CLI that don't implement swarm-related
features from depending on the swarm API types.

- Human readable description for the release notes

Go SDK: Deprecate `opts.PortOpt`, `opts.ConfigOpt` and `opts.SecretOpt`. These types were moved to the `opts/swarmopts` package.

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 59.31%. Comparing base (2eec746) to head (d0d91bb).
Report is 32 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5907      +/-   ##
==========================================
+ Coverage   59.26%   59.31%   +0.04%     
==========================================
  Files         357      357              
  Lines       29771    29782      +11     
==========================================
+ Hits        17645    17666      +21     
+ Misses      11153    11146       -7     
+ Partials      973      970       -3     

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