vendir support for "dot" paths without conflict

**Describe the problem/challenge you have**

vendir's requirement that paths do not overlap makes it difficult, or impossible, to create a directory structure that custom ytt modules can be found in if there are also environment specific yamls which need to be included.

consider the following.

* a _ytt_lib directory needs to be included from a git location
* some number of blobs of yaml (which i'll just call "modules" for now) need to be included
* some number of environment specific yaml one-off files need to be included.

The final directory structure that we're looking to create wants to look like this

```
├── config
   ├── _ytt_lib
   │   └── ambassador-mappings
   │   └── deployment.yaml
   ├── modules
   │   ├── base
   │   │   ├── defaults.yaml
   │   │   ├── deployment.yaml
   │   │   ├── namespace.yaml
   │   │   ├── service-account.yaml
   │   │   └── service.yaml
   │   ├── cdm-subscription
   │   │   └── deployment.yaml
   │   └── ...
   ├── secret.enc.yaml
   ├── terraform.yaml
   └── values.yaml
```

The above directory layout does not seem to be possible due to a constraint in vendir; `Expected to not manage overlapping paths: 'config/modules' and 'config'`

The vendir.yml file we're making using of looks roughly like the following.

```
---
apiVersion: "vendir.k14s.io/v1alpha1"
kind: "Config"
directories:
- path: "config"
contents:
- path: "."
git:
url: "https://gitlab.foo.bar/vss-cloud-accounts-service-infra.git"
ref: "v1.28.1"
includePaths:
- "k8s/envs/stg-01/secret.enc.yaml"
newRootPath: "k8s/envs/stg-01"
- path: "config/service"
contents:
- path: "modules"
git:
url: "https://gitlab.foo.bar/vss-cloud-accounts-service-infra.git"
ref: "v1.28.1"
includePaths:
- "k8s/modules/base/*"
- "k8s/modules/cdm-subscription/*"
- "k8s/modules/cloud-resources/*"
- "k8s/modules/cloud-account-credentials/*"
- "k8s/modules/csp/*"
- "k8s/modules/data-reconciler/*"
- "k8s/modules/flagger-canary/*"
- "k8s/modules/inventory-new-accounts/*"
- "k8s/modules/irsa-service-account-annotation/*"
- "k8s/modules/k8s-collector/*"
- "k8s/modules/kapp-config/*"
- "k8s/modules/legacy-changes/*"
- "k8s/modules/pod-disruption-budget/*"
- "k8s/modules/service-annotations/*"
- "k8s/modules/service-endpoints/*"
- "k8s/modules/wavefront/*"
newRootPath: "k8s/modules"
- path: "config/_ytt_lib"
contents:
- path: "ambassador-mappings"
git:
url: "https://gitlab.foo.bar/vss-cloud-accounts-service-infra.git"
ref: "v1.28.1"
includePaths:
- "k8s/modules/ambassador-mappings/*"
newRootPath: "k8s/modules/ambassador-mappings"
```

the first blob is the blob at issue because it wants to add content to ".", but "." conflicts with the overlay constraint.

**Describe the solution you'd like**

I'd like vendir to allow this use case similar to how `gilt` allows it. For example, the following `gilt` configuration produces the desired result.

```
---
- git: https://gitlab.foo.bar/vss-cloud-accounts-service-infra.git
version: v1.28.1
files:
- src: k8s/modules
dst: config/modules
post_commands:
- rm -rf ambassador-mappings
- src: k8s/modules/ambassador-mappings
dst: config/_ytt_lib/ambassador-mappings
- src: k8s/envs/stg-01/*
dst: config/
post_commands:
- rm -rf vendir.yml
```

but gilt itself has its own constraints in terms of functionality. constraints that vendir doesnt have.

**Anything else you would like to add:**

part of the issue here, as far as I understand things, has to do with the `ytt` requirement that the `_ytt_lib` directory must be a sibling of any yaml files that use content in that directory.

If this `ytt` constraint did not exist, then there would be a clear path forward in how to use vendir. We could create any number of sub-directories and then ytt could either be told where it's `_ytt_lib` is, or, `ytt` could walk up the tree until it found a suitable `_ytt_lib` to use.

---
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We are also happy to receive and review Pull Requests if you want to help working on this issue.

Start by reproducing the overlapping-path error with the vendir.yml configuration in the issue, then trace vendir's path-overlap validation. Define and test the intended behavior for a content path of "." alongside nested paths; done means the configuration succeeds and produces the requested config/modules, config/_ytt_lib, and environment-specific files without conflicts.

Written by the indexing model from the issue text.