common.dart is a large-and-growing file containing all shared code,
which makes it hard to navigate. To make maintenance easier, this splits
the file (and its test file) into separate files for each major
component or category.
- Replaces most explicit use of `fileSystem` with path construction using the `child*` utility methods
- Removes explicit passing of a filesystem to the commands; we're already passing a `Directory` for the
root where the tool operates, and we should never be using a different filesystem than that directory's
filesystem, so passing it was both redundant, and a potential source of test bugs.
