Feature request: support file relocation/rename

I would like `vendir` to strip/rename paths

```
- path: vendor/k8s
contents:
- path: eduk8s
git:
url: https://github.com/eduk8s/eduk8s
ref: 20.08.04.1
includePaths:
- resources/crds-v1/**/*
- resources/operator/**/*
```

and this ends up creating:

```
vendor/k8s/eduk8s/resources/crds-v1
vendor/k8s/eduk8s/resources/operator
```

but I would want:
```
vendor/k8s/eduk8s/resources/
```
everything just in this dir.

The use case is:

I’m collecting the k8s resource definitions from multiple sources to then process them with ytt and apply them with kapp, and have a consistent directory structure. When I get stuff from github releases I just get the files I need in the root of the path, when I get it this other way it ends in subdirectories, and hence dir layout is not really consistent

Start by tracing vendir's handling of the `path` and `includePaths` configuration for Git sources. Compare the generated directory tree with the examples in the issue, and define completion as placing the selected `resources/crds-v1` and `resources/operator` contents under the requested `vendor/k8s/eduk8s/resources/` layout.

Written by the indexing model from the issue text.