Reuse caba-dev in a parent folder, if one exists already

[rodlogic]

It would be great if cabal-dev could reuse an existing caba-dev/ folder found in a parent folder instead of always creating one in the current folder.

The main motivation is to be able to reuse a sandboxed cabal-dev for a number of projects without having to remember to set/unset cabal-dev's -s argument (this is specially the case if you have many of these independent uber projects). When first setting up my dev workspace for a given project, I would create a cabal-dev in the root folder (automatically if my root folder has a .cabal file or manually with something like cabal-dev init otherwise)

For instance, consider the following workspace structure for a given project:

/myprojws
     /cabal-dev
     /subsystem1
     /subsystem2
          /projectA
                /benchmark
                     Benchmark.cabal
    ...
     /subsystemN

I should be able to execute cabal-dev from /myprojws/subsystem2/projectA/benchmark and have caba-dev automatically reuse the /myprojws/cabal-dev/ repo since it is the first cabal-dev up the path chain.

When a conflict arises, cabal-dev could generate a new cabal-dev/ folder in the benchmark project and install there only the dependencies that would conflict with the ones in /myprojws/cabal-dev.

It would be my job (once) to lay out the folder structure so that I get the right repo separation.

[srhb]

This would also make cabal-dev Just Work (tm) with haskell-mode and friends.

[srhb]

This would probably also require that the (broken?) behaviour be fixed that launching eg. cabal-dev ghci from a directory that does not have a cabal-dev directory will create it, but still fail.