The refactor in f70fc980 solved an
inconsistency in `buildAndInstallUnpackedPackage`. Namely, before the
refactor, we didn't pass `hsSetupBuildArgs` to the build invocations,
and afterwards we did.

The result is that build targets of (legacy) package `B`, that a package
`A` depends on, are now passed to the ./Setup build invocation of
package `B` (e.g. `./Setup build lib:openapi3` rather than just `./Setup
build`).

This means we no longer /build always all targets of a legacy-package/
(for example, the lib, the testsuites and the executables).
Instead only the required target (just the lib) will be built.

However, despite now passing the build args to `./Setup build`, we
weren't passing the same arguments to the `./Setup copy` invocation.
Therefore, `./Setup copy` would also try to copy targets of the package
that weren't built, resulting in a failure.

This fixes that failure by correctly passing the build targets to the
`./Setup copy` invocation, just like we do for the `./Setup build` one.

Fixes #9640

This adds 4 tests which test the new external commands feature:

* ExternalCommand - Tests the expected usage of external command invoked
  via cabal-install
* ExternalCommandSetup - Tests that the ./Setup interface does not
  support external commands (#9403)
* ExternalCommandEnv - Tests that environment variables are set and
  preserved appropiately  (#9402)
* ExternalCommandHelp - Test that `cabal help <cmd>` is interpreted appropiately (#9404)

* Restructures the `cabal init` command to fix historical
  issues. All flags are preserved.
  * Codebases for interactive and non-interactive flags
    are disentangled.
  * Data structures now exploit relevant stanza structure
    and formatters only care about stanza data
  * Heuristics and prompts have a pure and impure implementation.

* Sets default behavior to be `--interactive` as opposed to
  `--non-interactive`.

* Rewrites tests to achieve 98% coverage
  * Golden files now test every stanza individually
  * Every flag is covered by a unit test
  * Interactive, simple, and non-interactive workflows are
    covered.