provide the ability to reference another multi-package project from a multi-package project

[dasormeter]

Want to support the ability for a multi-package project to reference another multi-package project, within the packages directive

Example

Given two multi-package projects like so

multi-package-example
├── multi-package.yaml
├── interfaces
│   └── daml.yaml
├── main
│   └── daml.yaml
└── test
    └── daml.yaml

multi-package-example2
├── multi-package.yaml
├── interfaces
│   └── daml.yaml
├── main
│   └── daml.yaml
└── test
    └── daml.yaml

i want a multi-package.yaml one level above these which can reference both.

packages:
- ./multi-package-example
- ./multi-package-example2

And then the usual multi-package commands like dpm build --all should just work and recurse through all the packages under the multi package projects