…o generate all variants with the new mode generate-variants

…ass for the moment.

"verible::CmdPositionalArguments" class only supports these types so far: SV files, +define+<name>[=<value>], and +incdir+<dir>.

- Adding an interface function "AddDefineFromCmdLine" to use the macro added by +define+<foo> argument to "VerilogPreprocess".

- Added a feature to VerilogPreprocess which allows to store paths,
  That can be used later to find the SV file to include.
- The preprocessor tool takes these paths from the user,
  as a +incdir+<path>[+<another_path>] and set then in VerilogPreprocess.
- The included files macros and conditionals can be expanded and evaluated.
- Some limitations exists and were written as TODOs in place,
  need to open issues for these.

- The preprocessor tool preserves the white-spaces in the SV files.
- Changed the output of the -generate_variants and -multipecu from tokens (enum,text) into normal text.