Add flag to cabal to specify requiredInstall packages in modular solver.

[robinp]

In the modular solver at

cabal/cabal-install/Distribution/Solver/Modular/Solver.hs

Line 134 in 2146ea1

P.requireInstalled (`elem` [ mkPackageName "base"

, the list of wired-in packages is hardcoded.

When one wants to rebuild one of these packages (see mail thread about rebuilding base), the given package needs to be commented out, and cabal rebuilt locally.

It would be nice if the list of packages came from a flag (defaulting to the current set?), which users could override. Or, if it's needed for some reason, keep the list hardcoded but let some packages be removed with a flag.

[grayjay]

@robinp I rebased my branch from #3987 and made a PR. The flag always ignores the whole list of non-installable libraries, because cabal already has "installed" constraints. For example, cabal new-build --constraint="base installed" --install-base-libraries would allow cabal to install template-haskell but not base.

EDIT: The PR is #4227.

[grayjay]

Fixed by #4227.