The Stackage project provides a cabal.config which locks in a specific
package set.  This file should be shared between developers project so
that everyone is developing using the same versions of packages.

"GHC creates a subdirectory .hpc in the current directory, and puts HPC index (.mix) files in there..."

https://www.haskell.org/ghc/docs/7.4.2/html/users_guide/hpc.html

https://github.com/haskell/cabal/wiki/Package-Environments#sandbox-command