How do I debug a dependency package? If I have its source code

First fetch all the dependency packages into the vendor folder.

go mod vendor

Then, change the source code in that and build your project by specifying to look into vendor folder.

go build -mod=vendor

or

go run -mode=vendor myapp.go

You can use replace directive:

replace example.com/original/import/path => /your/forked/import/path

Go module fetches packages into $GOPATH/pkg/mod you can change the source code there or using the vendor option of go mod to pull the packages into vendor folder and then start coding there.