Failures on "unknown package" should list active repositories

[gbaz]

Suggested by @tomjaguarpaw in the HF slack. Joachim suggested that we might want to suppress this when active-repositories is the default. I think it wouldn't be too much line noise either way, especially since this error is not going to come up constantly, and when it does, the more context the better.

[ameya-deshmukh]

@gbaz is this still open for contributions? Would love to give it a shot :)

[gbaz]

Sure, go for it!

[gbaz]

To be a bit more explicit about the issue, here is the current behavior:

> cabal repl -b badpackage
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: fake-package-0 (user goal)
[__1] unknown package: badpackage (dependency of fake-package)
[__1] fail (backjumping, conflict set: badpackage, fake-package)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: fake-package, badpackage

So we have a failure in the solver due to an unknown package, and that's reported.

But note that the user may be in a cabal project where active-repositories is set to something nonstandard. So therefore, the package might be on hackage, but not in the nonstandard active-repositories set of repositories. Thus, it would be good to print out the repositories searched in the unknown package line, so a user might more easily notice this.

[ameya-deshmukh]

thank you, this helps!