Map Dart bindings file-per-class instead of file-per-package

[mahesh-hegde]

Current structure:

maps each java package to a dart file

Proposed strucutre:

Map each Java class to a dart file.
Provide an exporter file in every package.

It avoids the ambiguity where package can correspond to directory or file. It will always correspond to a directory.

Also, provide a _package.dart in each package dir, providing dart equivalent of Java's wildcard import.

Provide a single file option for small glue code. This should be intended for large libraries.

Single file is simple enough to be implemented with some renaming.

Interaction with pure dart bindings #773 .

It should be orthogonal. If #773 is implemented, all combinations should be supported.

output_structure = single_file | package_structure
bindings_type = c_based | dart_only

Status: Already working on it at a development branch. Still have to move to file-per-class mapping and test it. Might take a week or two.