Split the Package comment from the "boilerplate" comments

[dlsniper]

Actual behavior A clear and concise description of what the bug is.
The package comments are generated like this:

//	mockgen -destination demo/mock.go -package demo . Demoer
//
// Package demo is a generated GoMock package.
package demo

However, this is not correct, as it doesn't follow the package guidelines documented here: https://staticcheck.io/docs/checks#ST1000

Expected behavior A clear and concise description of what you expected to
happen.

I expect the generated code to be:

//	mockgen -destination demo/mock.go -package demo . Demoer
//

// Package demo is a generated GoMock package.
package demo

This would follow the guidelines and make the linter happy too.

To Reproduce Steps to reproduce the behavior

1. Run mockgen -destination demo/mock.go -package demo . Demoer on any interface/package
2. Observe the results

Additional Information

• gomock mode (reflect or source): source
• gomock version or git ref: v0.3.1-0.20231011042131-892b665398ec
• golang version: go version go1.21.3 linux/amd64
• staticcheck version: v0.5.0-0.dev.0.20230826160118-ad5ca31ff221

Triage Notes for the Maintainers

Here's an attempt at fixing the issue: #112.

[JacobOaks]

Closing since #112 was merged. Thanks!