Build constraints in generated files

[ARR4N]

Requested feature The ability to specify build constraints to be added with //go:build <constraints> in the generated files.

Why the feature is needed Generated mocks are intended for testing but are regularly in non _test.go files to allow for cross-package imports. This risks the leakage of test-only code into production, which can easily be prevented with build constraints (example).

(Optional) Proposed solution #191

[jaceee]

+1
I really need this. Thanks for the PR too, that's exactly what I need.

[Gris87]

+1
We need it too