Readable#fileName is being used incorrectly

[superaxander]

Multiple places across VerCors assume that Readable#fileName will be a valid path to an existing path. This is not always true. (for example if an input originates from stdin)

Places I've found so far:

• SilverToCol passes the path to the Silver parser and then creates a new RWFile in the origin of AST nodes using the path in Silver's SourcePosition class. In b5e997d I've changed it to use Readable#underlyingPath when that is not None but you can still get crashes if there is no underlyingPath.
• SerializeOrigin used fileName instead of underlyingPath (fixed in b5e997d)
• Output.scala also uses fileName but it also creates these fileNames itself (so we know there are no folders that can get cut off and this is probably fine)