Add support for flakes in subdirectories

[edolstra]

Currently flake.nix has to live in the top-level directory of a repository. Thus, there can be only one flake per repository. We could add a flake path to flake references to allow subdirectories (e.g. github:my-org/my-project?dir=foo/bar).

Whether this is desirable is not entirely clear (we may want to be opinionated about flake structure). Apparently NPM doesn't support this either and they survived just fine without it.

[zimbatm]

Yarn has added a concept of "workspaces" which does something like that. In a big monorepo where there are multiple projects, each project gets it's package.json and the workspace is used to group them all.

https://yarnpkg.com/lang/en/docs/workspaces/