Format generated files with goimports

[timebertt]

Fixes #393

Description
This PR changes mockgen to generate files that are goimports-formatted.

The actual change is in mockgen/mockgen.go, the other changes are generated.

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• Includes tests

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• Tests added.

Release Notes

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- Format generated files with goimports

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[timebertt]

@googlebot I signed it!

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CLAs look good, thanks!

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[timebertt]

cc @codyoss
Can you take a look, please?

[timebertt]

Ping
Can somebody check the PR, please?

[cvgw]

LGTM! Thank you for the contribution!