Some tools in GN toolchains are implemented via scripts. When the script changes, we want to call all of those actions out-of-date.

This could be implemented as the deps of a toolchain but that's very heavyweight, both conceptually (requiring a dummy setup toolchain and a separate file) and at runtime (dependency file gets added to every single build rule, the entuire build is invalidated when one tool changes).

For this case it would be better to have an "inputs" entry for individual tools that causes a dependency for every rule that the tool is used for on those files. Tools would list all the scripts and other inputs they require in this field.

Currently this is coming up because of the mac linker_driver tool, but is also theoretically present on everything implemented via gyp-*-tool.