move the core logic of mockgen out of `main` package to make mockgen importable

[sanposhiho]

Hi team.

What

It would be nice if the core logic of mockgen is placed in some package other than main. This makes mockgen importable.

main package should contain only CLI-related processes such as option parsing.

Why

Users can import mockgen features. (In Go, we cannot import main package.)

This allows users

• to customize mockgen to meet their own requirements.
• to generate mock files for gomock by their own tool.

...etc

For example, I am developing a tool, called gomockhandler, to manage mocks of gomock.
https://github.com/sanposhiho/gomockhandler

Users need to install mockgen to use gomockhandler and gomockhandler calls mockgen through exec.Command.
https://github.com/sanposhiho/gomockhandler/blob/master/internal/mockgen/sourcemode/runner.go#L68

However, unexpected behaviors may occur due to differences in mockgen behavior depending on which mockgen the user has installed.
Ideally, it would be nice to be able to generate mocks without using user-installed mockgen and we can achieve that if we can import mockgen.

[sanposhiho]

If you want, I can work on this.

[codyoss]

This seems similarish to #424. I am not against this this idea but it is a large task. The APIs for mockgen are not really meant to be used directly today and are not ergonomic. Much refactoring and a detailed design would need to be in place before a change like this landed. I would like to leave this open for a bit to see if there is more demand for something like this as it would be a large feature I think.