Matthew Pickering authored Oct 12, 2022 and Francesco Gazzetta committed May 28, 2023

There are several parts to this patch which are logically distinct but
work together to support the overal goal of starting a GHCi session with
multiple packages loaded at once.

1. When a user writes "cabal repl <target>" then if the user is using a
   compiler > ghc-9.4.* then we will attempt to start a multi-session
   which loads the selected targets into one multi-package session of
   GHC.
1a. The closure property states that in order to load components `p` and `q` into
    the same session that if `p` depends on `z` and `z` depends on `q`
    then `z` must also be loaded into the session.
1b. Only inplace packages are able to be loaded into a multi session (if a component
    `z` exists then it is already made into an inplace package by
    cabal). Therefore cabal has already engineered that there is source
    code locally available for all packages which we will want to load
    into a session.

2. The solver is unmodified, the solver is given the repl targets and
   creates a build plan as before. After the solver is completed then in
   `setRootTargets` and `pruneInstallPlan` we modify the install plan to
   enforce the closure property and mark which dependencies need to be
   promised.

   * Mark the current components as `BuildInPlaceOnly InMemory`, which
     indicates to the compiler that it is to be built in a GHC
     multi-session.
   * Augment the component repl targets to indicate that components
     required by the closure property (in addition to normal targets)
     will be loaded into the repl.
   * Modify the dependency edges in `compLibDependencies` to indicate
     which dependencies are the promised ones (which is precisely
     components which are `BuildInPlaceOnly InMemory` build styles).
     This is the field which is eventually used to populate the
     `--dependency` argument to `./Setup configure`.

Fixes #8491