Warn about overly many source-repository stanzas

[philderbeast]

Can we have cabal check warn if there is more than one source-repository head or more than one source-repository this stanza?

I ran a quick cabal check and we can have as many source-repository stanza as we please. The following passes cabal check;

$ git diff
diff --git a/cabal-install/cabal-install.cabal b/cabal-install/cabal-install.cabal
index f842d4d31..afc1101b9 100644
--- a/cabal-install/cabal-install.cabal
+++ b/cabal-install/cabal-install.cabal
@@ -26,6 +26,23 @@ source-repository head
   location: https://github.com/haskell/cabal/
   subdir:   cabal-install
 
+source-repository head
+  type:     git
+  location: https://gitlab.com/haskell/cabal/
+  subdir:   cabal-install
+
+source-repository this
+  type:     git
+  location: https://github.com/haskell/cabal/
+  tag:      v3.11.0.0
+  subdir:   cabal-install
+
+source-repository this
+  type:     git
+  location: https://gitlab.com/haskell/cabal/
+  tag:      v3.11.0.0
+  subdir:   cabal-install

Originally posted by @philderbeast in #9655 (comment)

[ffaf1]

Interestingly enough, PackageDescription allows more SourceRepos:

cabal/Cabal-syntax/src/Distribution/Types/PackageDescription.hs

Line 121 in c3a9dd7

, sourceRepos :: [SourceRepo]

I wonder whether there is a legitimate use-case for multiple repos.

[ulysses4ever]

It's pretty clear to me that at least two should be allowed, as long one is head and one is this (not that I think it's a great idea to have both). If we could emit a soft warning on the repetitive head or this, that would be an (admittedly slight) improvement.