Improve list parser

[maksbotan]

Current parser for lists, like packages: in cabal.project.local,
fails to process some input, for example:

packages:
foo/foo.cabal
, bar/bar.cabal
, baz/baz.cabal

This commit simplifies the parser by using sepBy directly and by
handling indented fields more uniformly.

[Avi-D-coder]

See the test failure.

It also needs to parse newline delineated lists and mixed newline comma lists.
Cabal will except:

packages:
         ./
         ghcide

[maksbotan]

Yeah, I'm already on it.

[maksbotan]

While reading Cabal sources I missed that it first parses fields into a tree and then parses its parts, that's why parseOptCommaList does not have to deal with indices.

This commit should work. I've also added a testcase.

[Avi-D-coder]

Thanks,
I'll publish new version in the next few days

[maksbotan]

Thanks!
The tool is very useful, by the way :)