support cabal sandbox, fixes #62

unfortunately this breaks the support for cabal-dev, so more work is
required.
rename CabalDev.hs to Sandbox.hs

this file is no longer about CabalDev, but about sandboxing support.
describe the two sandbox types

we plan to support both cabal-dev and cabal-sandbox.
find the sandbox path from the sandbox description
uninline cabalSandboxDir
validate package path using a sandbox description
find the package path from the sandbox description

I also had to change a few function names to avoid naming conflicts.
support both cabal-dev and cabal-sandbox
Merge branch 'sandbox-support' into develop