Packager: use of current directory as default input path for packager causes confusion

Most of the javafxpackager commands do not require source files/directory to be explicitly provided.
It is not provided they assume all files in the current directory is the input.

This easily confuse developers who may run packager in the wrong folder and then observe a lot of files copied over.

We better require at least one of -srcdir or -srcfiles to be specified.

This part of behavior (mandatory option or not) is currently not documented in the packaging docs and may be it is worth to update docs.