Improve speed of `cargo dev fmt` (#14862)

This stops using `cargo fmt` and instead calls rustfmt directly with the
list of all files.

All `cargo fmt` does is find the crate roots and passes the edition from
`cargo.toml`. Since the edition is set in `rustfmt.toml` for the test
files and we're already iterating through all the files this is not
needed.

`--skip-children` is used since we already pass all the files, so the
automatic detection isn't buying us anything other than running slower.

~Second commit~ (part of the first commit now) is a change to only use
the `ignore` option in `rustfmt.toml` rather than having a way in `cargo
dev fmt` to ignore files.

r? @samueltardieu

changelog: none

0 files changed, 0 insertions, 0 deletions