Add an option to emit a single dependencies file (*.d) for the entire build

[roopekv]

It seems that there isn't currently a proper way to emit a single Make-compatible dependencies file (*.d) for the entire build. This would be a useful feature for integration with build systems.

Example of a problem this would help to solve:
https://forums.swift.org/t/changes-to-c-headers-not-causing-swift-code-to-recompile/78248

There has been some demand for this feature in the past as well:
https://forums.swift.org/t/single-depfile-for-swiftpm-target/30169