Backport #11237: Fix two issues when starting the repl

[mergify]

Issue 1: In 3.14 there was a poor error when starting a repl in a project context without any targets. In 3.16, this error regressed, so cabal just exited cleanly.
Issue 2: The repl was broken when started from a global context.

Issue 1 is fixed by checking to see if there are any user targets, and issuing a proper error if there are none.

Issue 2 is fixed by specifying the correct fake target which is constructed when starting the repl in the global context.

Both are reported in #11107 and fixed in this patch.

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This is an automatic backport of pull request #11237 done by [Mergify](https://mergify.com).