feat: use caching for `directories` entries

**Describe the problem/challenge you have**

**See discussion below: I changed this request to a "implement caching request"**

Vendir is really slow if you clone multiple repos and sources.

**Describe the solution you'd like**

It would be nice to have a cache feature on entries in `directories` such that vendoring is **faster**

So in the below: `vendir` would cache the sources from `&ref` once and then the copying of the files for all entries would speed up tremendously.

```yaml
apiVersion: vendir.k14s.io/v1alpha1
kind: Config
directories:

- path: ./bla
contents: &ref
- path: .
git:
url: https://gitlab.com/data-custodian/custodian.git
ref: main
depth: 1
newRootPath: tools


- path: /a
contents: *ref

- path: /b
contents: *ref

- path: /c
contents: *ref

- path: /d
contents: *ref
```

> [!NOTE]
> Vendir should have a two step process, first download all sources (for which it does not have a cache entry)
> next distribute all files in the `directories` entries. This will also make parallel processing trivial for the second step, parallel execution for the first step might be more tricky depending on how the download tools work.

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