Track private executable inter-component dependencies (Was: Document new `scope` field in manual)

A new field `scope` was added via #4501

However, I don't see it mentioned anywhere in http://cabal.readthedocs.io/en/latest/cabal-projectindex.html so I have to assume it hasn't been documented yet.

I'm also wondering if this is only relevant to the old non-`new-build` paradigm, given how differently `new-build` works (executables live in the cabal nix store, and we'd probably just symlink to them, to create specific `bin/`-folder views into the store), or alternatively, how this is supposed to interact w/ the nix-style `new-build` paradigm.

/cc @DanielG

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UPDATE:

Some bikeshedding names for the new property:

`(dist|runtime|libexec)-(tool-|exe-|)-depends`

Also, we need to implement some new `Paths_` lookup facilities; as we can *not* assume that things are locatable via the `libexec` folder due to the per-component nature of cabal's nix-style store.

Start by reviewing the `scope` field introduced in #4501 and the Cabal project index linked in the issue. Trace how private executable dependencies should work for both the old paradigm and `new-build`, including per-component store locations and the proposed `Paths_` lookup facilities. Done means the dependency behavior and naming are settled and the relevant documentation and implementation are updated.

Written by the indexing model from the issue text.