Add objc_library binary to outputs of apple_framework_packaging rule when virtualized frameworks feature is enabled

[jschear]

This fixes an issue where building an apple_framework_packaging rule with the VFS feature enabled wouldn't trigger objc compilation for the framework.

When the VFS feature is disabled, the binaries from the objc_library and swift_library targets are merged using libtool, and that binary is included in the outputs of the apple_framework_packaging rule (see packaging action).

When the VFS feature is enabled, all of the packaging actions are skipped, and the binaries from the objc_library and swift_library targets aren't merged. One of the input binaries is arbitrarily included in the rule's output files -- for a target containing swift and objc sources, this would be the swift_library binary.

The PR adds both input binaries to the rule outputs when the VFS feature is enabled, so building an apple_framework_packaging rule triggers both objc and swift compilation actions.

[luispadron]

Makes sense thank you!

[thiagohmcruz]

LGTM! Thx for digging on this one!

[gyfelton]

before this change, swiftmodule_out equals to [swiftmodule_in] when VFS is enabled
with this PR, swiftmodule_out equals to swiftmodule_in instead when VFS is enabled. Am I understanding this diff correctly?

[jschear]

I think swiftmodule_out is equal to swiftmodule_in both before and after this change.

Before:

swiftmodule_out = _framework_packaging_single(ctx, "swiftmodule", [swiftmodule_in], swiftmodule_out, framework_manifest)

def _framework_packaging_single(ctx, action, inputs, output, manifest = None):
    outputs = _framework_packaging_multi(ctx, action, inputs, [output], manifest = manifest)
    return outputs[0] if outputs else None

def _framework_packaging_multi(ctx, action, inputs, outputs, manifest = None):
   # ...
    virtualize_frameworks = feature_names.virtualize_frameworks in ctx.features
    if virtualize_frameworks:
        return inputs
   # ...

_framework_packaging_multi just returns the inputs list, and _framework_packaging_single returns the first item of that list. Does that make sense?