In workspaces, `cargo fmt` only works in the top-level directory #8724

Describe the bug

$ cargo fmt
Failed to find targets
cargo-fmt 1.4.18
This utility formats all bin and lib files of the current crate using rustfmt.

USAGE:
    cargo fmt [FLAGS] [OPTIONS] [-- <rustfmt-options>...]

FLAGS:
        --all        Format all packages (only usable in workspaces)
    -q, --quiet      No output printed to stdout
    -v, --verbose    Use verbose output
        --version    Print rustfmt version and exit

OPTIONS:
        --manifest-path <manifest-path>      Specify path to Cargo.toml
        --message-format <message-format>    Specify message-format: short|json|human
    -p, --package <package>...               Specify package to format (only usable in workspaces)

ARGS:
    <rustfmt-options>...    Options passed to rustfmt

To Reproduce

1. cargo new outer && cd outer
2. cargo new inner
3. printf '[workspace]\nmembers = ["inner"]' >> Cargo.toml
4. cd src
5. cargo fmt

Expected behavior

cargo fmt should format all packages in the workspace, like it does from the top-level directory.

Meta

• rustfmt version: rustfmt 1.4.21-nightly (01f2ead 2020-09-04), but the problem is also present on rustfmt 1.4.18-stable (8157a3f 2020-07-15)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: cargo fmt

Reproduce the issue with the listed cargo new, workspace, and cargo fmt commands, especially from the inner src directory. Trace how cargo fmt discovers the workspace and packages when invoked below the top level; done means cargo fmt from that directory formats all workspace packages as it does from the workspace root.