Make a trailing colon for stanzas a parse failure

[9999years]

Here's a mistake I make semi-regularly:

source-repository-package:
    type: git
    location: https://github.com/parsonsmatt/foundation
    tag: 688c32ccd9a951bc96dd09423a6e6684f091d510
    subdir: basement
    subdir: foundation

Cabal treats this as a warning, so it prints:

Warning: cabal.project: Unrecognized field
'source-repository-package' on line 52

This is fine (if you already know the mistake you've made, at least!), but it's very easy to miss amidst lots of output.

I often re-run cabal when I see a ton of output to attempt to get a smaller error message. (Usually it works and I get an error message that's got less "compiling module such and such" noise in it.) However, re-running cabal will discard this warning entirely!

Let's make it a hard error instead. This is a backwards-compatibility break.

Now, it prints this error:

Error: [Cabal-7090]
Error parsing project file cabal.project:52:
'source-repository-package' is a stanza, not a field. Remove the trailing ':' to parse a stanza.

• Still needs tests!