Default to standard library when validating zip file

[aemengo]

Summary

Go stdlib zip format validation looks non-trivial. Might be worth delegating..

Output

Before

$ pack build sample-app --path target/sample-0.0.1-SNAPSHOT.jar --builder cnbs/sample-builder:bionic
ERROR: failed to build: invalid app path 'target/sample-0.0.1-SNAPSHOT.jar': app path must be a directory or zip

After

$ pack build sample-app --path target/sample-0.0.1-SNAPSHOT.jar --builder cnbs/sample-builder:bionic
...

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Resolves #1236