Projects: Outline the project namespace structure

[hojberg]

Projects should have guard railed namespace structure to make it easy for developers to setup and develop, deal with releases, prs and dependencies. The goal of this ticket is to finalize the structure.

Note that a strict structure is likely required for Unison Share to consume a project.

Rough draft:

▼ http
├── ☆ project
├── ▼ pullrequests
│   ├── ▻ hojberg-321
│   └── ▻ rlmark-324
├── ▢ README -> trunk/README
└── ▼ trunk
    ├── ▻ _external
    ├── ◇ Header
    ├── ▻ Header
    ├── ◇ HttpMethod
    ├── ▻ HttpMethod
    ├── ◇ HttpRequest
    ├── ▻ HttpRequest
    ├── ◇ HttpResponse
    ├── ▻ HttpResponse
    ├── ◇ HttpServer
    ├── ▻ HttpServer
    ├── ◇ HttpStatus
    ├── ▻ HttpStatus
    ├── ▢ README
    ├── ◇ RequestBody
    ├── ▻ RequestBody
    ├── ◇ Url
    ├── ▻ Url
    ├── ◇ example
    ├── ● patch
    └── ◇ server

Legend:   
  ▼ : expanded namespace
  ▻ : collapsed namespace
  ◇ : term or type
  ▢ : doc
  ☆ : project
  ● : patch

The project definition design is covered by https://github.com/unisonweb/unison/issues/2288