enable finding alternative builders nested under Jib projects

[balopat]

Fixes #3654.

Now the output of skaffold init --analyze --skip-deploy --XXenableJibInit | jq on the java-guestbook Cloud Code sample is:

{
  "builders": [
    {
      "name": "Jib Maven Plugin",
      "payload": {
        "path": "pom.xml"
      }
    },
    {
      "name": "Jib Maven Plugin",
      "payload": {
        "path": "pom.xml"
      }
    },
    {
      "name": "Docker",
      "payload": {
        "path": "backend/Dockerfile"
      }
    },
    {
      "name": "Docker",
      "payload": {
        "path": "frontend/Dockerfile"
      }
    }
  ]
}

note, the repeated pom.xml is still due to GoogleContainerTools/jib#2262

[codecov]

Codecov Report

Merging #3671 into master will increase coverage by 0.02%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/builders.go	94.36% <100%> (+0.03%)	⬆️
...affold/kubernetes/portforward/kubectl_forwarder.go	68.29% <0%> (+2.43%)	⬆️