Multi-input generator support

[lhearachel]

In one of my project's use cases, I must compile an archive from a set of generated binary files. Some of these binary files are built using a tool which accepts both a PNG and an associated JSON file as input. It is not unusual for a user to modify either of these files, and a modification to either would need to trigger re-generation of the binary file (and thus, also the archive build).

I can accomplish a facsimile of this using custom_target, but I'd really like to be able to use generator for it, since our archiving utility accepts a path as input. It would be convenient, then, to make use of a custom_target's private intermediate directory. i.e., I should be able to do the following:

some_generator = generator(some_program,
    arguments: [
        '@INPUT0@',
        '@OUTPUT@',
        '-metadata', '@INPUT1@',
        '@EXTRA_ARGS@'
    ],
)

input_files = [
    files('file_1.png', 'file_1.json'),
    files('file_2.png', 'file_2.json'),
    # ...
]

archive_file = custom_target('archive',
    output: 'archive',
    input: some_generator.process(input_files), # produces 'file_1.bin', 'file_2.bin', etc.
    command: [
        archive_exe,
        '--directory', '@PRIVATE_DIR@',
        '--pack', '@OUTPUT@',
    ],
)