Ability to customize package_prefix instead of using mock_

[yuamera]

Requested feature

When using mockgen, when no package name is specified, you want to be able to customize the prefix or empty string instead of using mock_

Why the feature is needed

When I use mockgen to generate code for the interface generated by protobuf, I just want to be able to put them in the same package without knowing what the package name is. Especially when protobuf specifies a unique go package name

Proposed solution

Add a -package_prefix parameter to set the prefix instead of hard-coding mock_ ?