Improve list parser

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 adapting the code from
`cabal-install` itself:

https://github.com/haskell/cabal/blob/06c3eff242154cdf6b3d1bbaffcfc1a5cbea9da8/cabal-install/Distribution/Deprecated/ParseUtils.hs#L555

Author: maksbotan

src/Hie/Cabal/Parser.hs

@@ -92,19 +92,17 @@ parseString = parseQuoted <|> unqualName
 unqualName :: Parser Text
 unqualName = takeWhile1 (not . (\c -> isSpace c || c == ','))
 
+-- | Comma separated list.
 parseList :: Indent -> Parser [Text]
 parseList i = items <|> (emptyOrComLine >> indent i >> items)
   where
-    items = do
-      skipMany tabOrSpace
-      h <- parseString
-      skipMany tabOrSpace
-      skipMany (char ',')
-      t <-
-        items
-          <|> (skipToNextLine >> indent i >> parseList i)
-          <|> pure []
-      pure $ h : t
+    items = sepBy parseString (skipSpace *> char ',' *> skipSpace)
+
+-- | Comma or space separated list.
+parseOptCommaList :: Indent -> Parser [Text]
+parseOptCommaList i = items <|> (emptyOrComLine >> indent i >> items)
+  where
+    items = sepBy parseString ((skipSpace *> char ',' *> skipSpace) <|> (space *> skipSpace))
 
 pathMain :: Indent -> [Text] -> Text -> [Text] -> [Text] -> Parser [Text]
 pathMain i p m o a =
@@ -185,4 +183,4 @@ indent i = do
   if c >= i then pure c else fail "insufficient indent"
 
 extractPkgs :: Parser [T.Text]
-extractPkgs = join . catMaybes <$> many' (Just <$> field 0 "packages" parseList <|> (skipToNextLine >> pure Nothing))
+extractPkgs = join . catMaybes <$> many' (Just <$> field 0 "packages" parseOptCommaList <|> (skipToNextLine >> pure Nothing))