REPL command in project requires a target

[philderbeast]

Fixes #10527 and #9983. Doesn't allow an empty list of targets with a project context.

With a project, the REPL command requires a target. If one is not given then a message is shown explaining this and naming the project if the --project-file option was given (but not when the default 'cabal.project' project name is used implicitly). We're not yet able to list project targets so in the meantime, the messages lists the packages of the project.

• When the implicit default cabal.project is used:

$ cat cabal.project
packages: pkg-one pkg-two

$ cabal repl
Error: [Cabal-7076]
With a project, the REPL command requires a single target. The packages in this
project are:
 - pkg-one
 - pkg-two

• When the --project-file option is used, the file name is included:

$ cat some.project 
packages: pkg-one pkg-two

$ cabal repl --project-file=some.project
...
Error: [Cabal-7076]
With a project, the REPL command requires a single target. The packages in this
project, 'some.project', are:
 - pkg-one
 - pkg-two

• When the project has no packages, this is mentioned in the message:

$ cat empty.project

$ cabal repl --project-file=empty.project
...
Error: [Cabal-7076]
With a project, the REPL command requires a single target but there are no
packages in this project, 'empty.project', to choose a package (library) or
other component from as the target for this command.

• Before the fix the message mentioned a fake-package-0. This was confusing:

$ ~/.ghcup/bin/cabal-3.12.1.0 repl --project-file=some.project
...
Error: [Cabal-7076]
Internal error when trying to open a repl for the package fake-package-0. The
package is not in the set of available targets for the project plan, which would
suggest an inconsistency between readTargetSelectors and resolveTargets.

• Earlier cabal-install:exe:cabal versions mentioned using all as the target but this won't work for the REPL command:

$ ~/.ghcup/bin/cabal-3.10.3.0 repl --project-file=some.project
Error: cabal-3.10.3.0: No targets given and there is no package in the current
directory. Use the target 'all' for all packages in the project or specify
packages or components by name or location. See 'cabal build --help' for more
details on target options.

---

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• Tests have been added.