simplify mockgen usage for private interfaces

[marten-seemann]

I finally figured out a way to get rid of the horrible bash script currently used for mock generation: https://github.com/quic-go/quic-go/blob/af517bdef10a483bacd5eb96e67ebca4e4e151b2/mockgen_private.sh. libp2p/go-libp2p#2248 (comment) is what triggered this.

The trick is to export the type using a type alias, but to hide this behind a build flag:

//go:build gomock || generate

type MyInterface = myInterface

mockgen can then be called with the -build_flags="-tags=gomock" option to set the required build tag. And to be able to have the go generate command in the same file, the generate build flag is added to the file as well (go generate sets the generate build flag automatically).

[codecov]

Codecov Report

Merging #3769 (2a85e4c) into master (379e7ec) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3769   +/-   ##
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  Coverage   84.23%   84.23%           
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  Files         142      142           
  Lines       14084    14084           
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  Hits        11863    11863           
  Misses       1801     1801           
  Partials      420      420           

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