Warn about overly many `source-repository` stanzas

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`;

```diff
$ 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 https://github.com/haskell/cabal/issues/9655#issuecomment-1907990554_