Simplify output path generation

[mike-lischke]

This is a new version of PR #1829 from 19 April.

The twisted way to create a javac like output path breaks many use cases, including very simple ones with lexer + parser grammar in a relative subdir (where parser generation doesn't find the lexer tokens). Additionally, not all projects prefer to have their generated files in an output folder which is comprised of the given output path and the subdir given for the grammar files. The check for absolute paths as decision criterion is a weak one anyway - it makes simple usage difficult and doesn't deal with path expansion on all platforms.

If a javac like output path is required it should be easy to construct this upfront and provide it as the output path parameter, instead of implicitely creating it. This patch simplifies things without any WTF moments because generated files end up at unexpected locations. It now can deal with simple cases (no output, no grammar subdir) up to complicated cases (nested and/or absolute output dir, multiple grammar subfolders).

If more than one output folder is wanted run the tool multiple times with different output path settings and add the lib option if you need to reference previously created files.

It might look like a disadvantage that now generation of parser files ends up at a different path when run with relative grammar subfolders, but actually that was broken anyway so I don't expect anyone having used that (but instead either absolute paths or no subfolders at all).

Fixes #1087, #753, #638, tunnelvisionlabs/antlr4ts#306, tunnelvisionlabs/antlr4ts#303