Make it easy to get the sha256sum of an input_files file

In tor-browser-build we use var/gradle_dependencies_version to name the directory containing the gradle dependencies, created from the file gradle-dependencies-list.txt. It would be better if it was named from the checksum of the gradle-dependencies-list.txt file, to avoid cases when we forget to update var/gradle_dependencies_version. However it is not easy to get the checksum of this file.

To make that easier I think we could add a feature to get the sha256sum of an input_files file, for example with something like [% c("input_files_sha256sum_by_name/$name") %].