feat(ninja): Support depends in generator.process

[lhearachel]

Overview

This adds support to specify additional dependencies during invocation of generator.process, which will be merged with the list of dependencies for the generator itself to produce a full dependency mapping for each output target. This can be useful for creating a single generator for processing a given type of input file where some cases may require additional targets to be built before some subset of them can be used.

Motivation

Consider the following illustration:

generated_constants = custom_target(...)
gen = generator(
    script_sh,
    arguments: [ '@INPUT@', '@OUTPUT@' ],
    depends: generated_constants,
    output: '@BASENAME@',
    depfile: '@BASENAME@.d',
)

custom_target(
    'some_binary_file.bin',
    output: 'some_binary_file.bin',
    input: gen.process(files(...))
    command: [ ... ],
)

more_constants = custom_target(...)
custom_target(
    'another_binary_file.bin',
    output: 'another_binary_file.bin',
    input: gen.process(files(...), depends: more_constants),
    command: [ ... ],
)

In this example, the another_binary_file.bin target accepts some input files which cannot be built correctly without more_constants. However, the generator itself does not need to have this dependency, as some_binary_file.bin's input files do not require these to be built, and thus they should not be flagged as dirty if more_constants is dirty.

Testing

The test case in common is functional for the Ninja backend. I will need some help understanding the mental model for the VSCode and XCode backends to make similar changes, as I have never worked with them before.

[dcbaker]

You have my ack for this idea. This MR also looks really nice.