Feature request: support in-project SDKs #294

There's currently no good way to embed an sdkcraft definition in existing projects. A common SDK tree will look like:

.
├── hooks
│   └── setup-base
├── README.md
├── sdkcraft.yaml
└── tests
    ├── main
    │   └── launch
    │       ├── consumer.cpp
    │       ├── task.yaml
    │       └── workshop.yaml.in
    └── spread.yaml

5 directories, 7 files

At least hooks and sdkcraft.yaml need to be relocatable so this can coexist within the upstream source repo and reference source: ..

The .workshop/<sdk> could possibly be used here, with sdkcraft taking either the SDK name, or the full path, as argument.

Alternatively, following other *crafts, sdk/sdkcraft.yaml, .sdk/sdkcraft.yaml.