Confusion about what `path` actually represents

My goal is to synchronize a few Git repositories in a certain local directory.

It seems `path` can be a completely different object if it's under `directories` or `contents`. Here's my journey.

One cannot specify `.` as the directory and add entries in `contents`:

```
$ cat vendir.yml
apiVersion: vendir.k14s.io/v1alpha1
kind: Config

directories:
- path: '.'
contents:
- path: examples
git:
url: https://github.com/kubernetes/examples
ref: master

$ vendir sync
vendir: Error: Parsing resource config 'vendir.yml':
Unmarshaling config:
Validating config:
Validating directory '.' (0):
Expected path to not be one of '/', '.', '..', ''
```

And it's also not possible to specify a list of directories directly under `directories`:

```
$ cat vendir.yml
apiVersion: vendir.k14s.io/v1alpha1
kind: Config

directories:
- path: examples
git:
url: https://github.com/kubernetes/examples
ref: master

$ vendir sync
Lock config

apiVersion: vendir.k14s.io/v1alpha1
directories:
- contents: null
path: examples
kind: LockConfig

Succeeded

$ ls -l examples/
total 0
```

Even with `contents` it doesn't work:

```
$ cat vendir.yml
apiVersion: vendir.k14s.io/v1alpha1
kind: Config

directories:
- path: examples
contents:
git:
url: https://github.com/kubernetes/examples
ref: master

$ vendir sync
vendir: Error: Parsing resource config 'vendir.yml':
Unmarshaling config:
Unmarshaling config: error unmarshaling JSON: while decoding JSON: json: cannot unmarshal object into Go struct field Directory.directories.contents of type []config.DirectoryContents
```

The Eureka moment: `path` is not the same kind of object with everywhere.

Finally, the ~~workaround~~ solution is to define another self-referencing `.` path. Now, this works:

```
$ cat vendir.yml
apiVersion: vendir.k14s.io/v1alpha1
kind: Config

directories:
- path: examples
contents:
- path: .
git:
url: https://github.com/kubernetes/examples
ref: master

$ vendir sync
Fetching: examples + . (git from https://github.com/kubernetes/examples@master)

--> git init
[...]

Lock config

apiVersion: vendir.k14s.io/v1alpha1
directories:
- contents:
- git:
commitTitle: 'Merge pull request #497 from humblec/glusterfs-deprecation...'
sha: fb9ac11f42d665ae47cddbaea15c1f112c5f0464
path: .
path: examples
kind: LockConfig

Succeeded
```

A few thoughts:
- Maybe `contents` should actually be `subdirectories`, but still subdirectories would have a different schema from directories (confusing)
- Documentation could be changed to somehow mention that `directories` is just a local structure used to store `contents` and that a `content` is the actual thing you want to synchronize. I assume most people will be familiar with rsync and how it synchronizes remote directories with local directories but that's not how vendir works.