fix: check overlapping paths separately for directories and contents

[Zebradil]

This PR fixes the logic of the overlapping paths check. In the original versions, paths of directories were joined with paths of their contents before the check. This allowed the following configuration to be accepted:

apiVersion: vendir.k14s.io/v1alpha1
kind: Config
directories:
  - path: charts
    contents:
      - path: foo
        inline:
          paths:
            foo.txt: file-content
  - path: charts
    contents:
      - path: bar
        inline:
          paths:
            bar.txt: file-content

The result of vendir sync would be:

❯ vendir sync
Fetching: charts + foo (inline)

Fetching: charts + bar (inline)

Lock config

apiVersion: vendir.k14s.io/v1alpha1
directories:
- contents:
  - inline: {}
    path: foo
  path: charts
- contents:
  - inline: {}
    path: bar
  path: charts
kind: LockConfig

Succeeded

❯ exa -T
.
├── charts
│  └── bar
│     └── bar.txt
├── vendir.lock.yml
└── vendir.yml

One of the contents is overwritten by the other.

With this PR, having the same paths for two directories will result in an error.

[Zebradil]

Wrong branch, will create a new PR (#343).