support overriding directories within pulled dependencies via later directory configs

[cppforlife]

for example:

• pull in config/_ytt_lib/github.com/cloudfoundry/cf-for-k8s
• override config/_ytt_lib/github.com/cloudfoundry/cf-for-k8s/config/_ytt_lib/github.com/pivotal/kpack with something else.

[osterman]

@cppforlife would you say this relates kind of to our use case in #29 ?

[cppforlife]

nah dont think so. this is more about overriding directories wholesale (with an updated dependency) whereas #29 is more about modifying downloaded dependency. there is some relationship but prolly a bit different.

[osterman]

whereas #29 is more about modifying downloaded dependency.

Oh no, definitely not what I wanted to imply. We don't want to modify any file downloaded. We just want to keep files in the directory tree that did not originate at the source and not have those erased.

[cppforlife]

We don't want to modify any file downloaded

what i meant is that we see directory represented via a source as a single asset. so any modification to that directory (eg copying over temp files like *.tf.json) is "modifying asset".