PParser.hs: Add |filename| parser

As it stands, it reads in either a (possibly namespaced) identifier,
which was the old behavior, or a double-quoted string, which is the new
behavior.

This could be improved by actually testing that it's returning a filename,
but I'm clumsy with the parser abstraction, and it wasn't clear to me
from the Trifecta docs I looked through how to pass it through
System.FilePath.takeFileName and then enforce that it was a non-empty string.

src/Pkg/PParser.hs

@@ -62,6 +62,14 @@ pPkg = do
     eof
     return st
 
+
+-- | Parses a filename.
+-- |
+-- | Treated for now as an identifier or a double-quoted string.
+filename :: (MonadicParsing m, HasLastTokenSpan m) => m String
+filename = try $ (token $ stringLiteral <|> (fst <$> identifier)) <?> "filename"
+
+
 pClause :: PParser ()
 pClause = do reserved "executable"; lchar '=';
              exec <- fst <$> iName []