All projects should depend on a central packages.dhall in repo root dir

[epost]

Update 2019-08-29: This is now a Spago feature, see here.

TODO: think about whether we want this, now, or whether we should centralise our package defs in another way, because we may be splitting up this monorepo into separate repos.

• create /packages.dhall
• gather all package overrides, additions, etc from the various subprojects into that file
• point spago builds to it
  • make sure Marco's latest stbx-service-rest contributions do the same

Update: tried some of this, but the way I did it leads to nasty cyclic references:

erik@munin:~/dev/statebox/studio/studio (198/central-packages-dhall) $ npm run build

> statebox-studio@1.0.0 build /Users/erik/dev/statebox/studio/studio
> spago build

spago:
↳ ./spago.dhall
  ↳ ./packages.dhall
    ↳ ./../packages.dhall
      ↳ ./../halogen-tree-menu/spago.dhall
        ↳ ./../halogen-tree-menu/packages.dhall

Cyclic import: ../packages.dhall

1:            ../packages.dhall
2:

One workaround may be to not define the Spago-subprojects in their respective subdirs, but to define them in the central /packages.dhall instead. Have to think about whether this is desirable or whether we can come up with something nicer.

WIP notes containing more thoughts on how to do this: https://hackmd.io/NXJG84J9Shirel6KsCrrtA

[epost]

This is now a Spago feature, see mention here purescript/spago#301

[epost]

Fixed thanks to new Spago version