vendir capable of sync'ing files to its current-working-directory

**Describe the problem/challenge you have**
[A description of the current challenge that you are experiencing.]
i want to compose together a single repo using bits from other repos.

Among those other bits are top-level files like .pre-commit-config.yaml, .tflint, .isort, etc

**Describe the solution you'd like**
[A clear and concise description of what you want to happen. If applicable a visual representation of the UX.]
When I read the spec of vendir, intuitively it seems like vendir would support this.

for instance,

```
directories:
- path: .
contents:
- path: README
git:
url: git@github.com:org/repo.git
ref: c26b9b
includePaths:
- README.md
```

However, when I provide it such a config, I get the following.

```
Error: Parsing resource config 'vendir.yml': Unmarshaling config: Validating config:
Validating directory '.' (1): Expected path to not be one of '/', '.', '..', ''
```

I think the current assumption for the feature-set of the product is that vendir assumes control of everything in its CWD, so supporting something like this would cause it to stomp on existing files or something.

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

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