Reorganise repository to facilitate a hie-bios cradle

[mpilgrem]

A hie-bios cradle maps (a) filepaths to source code to (b) the buildable components of a package's Cabal file, in order to assist tools like HLS (Haskell Language Server).

In the case of Stack, currently, ./src contains source code for both components stack:lib and stack:test:stack-test (in ./src/test). It would be better for each component to have a unique filepath prefix.

I propose to move stack:test:stack-test to ./tests/unit and to move stack:exe:stack-integration-test to .tests/integration. This should not affect any of the pending pull requests.