@rules_rust//:rustfmt tool fails on crates with generated sources

[UebelAndre]

Any target which has a generated module (source file) will cause rustfmt to fail. The following error is produced on #2255

~/Code/rules_rust (rustfmt ✗) bazel run //:rustfmt
INFO: Analyzed target //:rustfmt (1 packages loaded, 12 targets configured).
INFO: Found 1 target...
Target //tools/rustfmt:rustfmt up-to-date:
  bazel-bin/tools/rustfmt/rustfmt
INFO: Elapsed time: 0.463s, Critical Path: 0.37s
INFO: 5 processes: 1 internal, 4 darwin-sandbox.
INFO: Build completed successfully, 5 total actions
INFO: Running command line: bazel-bin/tools/rustfmt/rustfmt
 checking cached actions
 checking cached actions
 checking cached actions
 checking cached actions
 checking cached actions
Loading: 0 packages loaded
Loading: 181 packages loaded
Loading: 0 packages loaded
Loading: 4 packages loaded
Loading: 0 packages loaded
INFO: Empty results
Loading: 0 packages loaded
Loading: 0 packages loaded
Loading: 26 packages loaded
Error writing files: failed to resolve mod `generated`: /Users/user/Code/rules_rust/test/rustfmt/srcs/generated/generated.rs does not exist

[UebelAndre]

rust-lang/rustfmt#5962 appears to be a relevant rustfmt issue.