Fix root detection in clippy for bin crates

[djmarcin]

Prior to this PR, bin crates with more than one file in srcs would fail with this error:

$ bazel build --aspects=@io_bazel_rules_rust//rust:rust.bzl%rust_clippy_aspect --output_groups=clippy_checks //...
INFO: Invocation ID: c3647be4-e5fe-4f90-a1a4-c9a84c17c45c
ERROR: /home/user/repo/BUILD:5:12: in @io_bazel_rules_rust//rust:private/clippy.bzl%rust_clippy_aspect aspect on rust_binary rule //foo:bar:
Traceback (most recent call last):
        File ".../io_bazel_rules_rust/rust/private/clippy.bzl", line 53, column 26, in _clippy_aspect_impl
                root = crate_root_src(ctx.rule.attr, srcs = rust_srcs)
        File ".../io_bazel_rules_rust/rust/private/rust.bzl", line 115, column 13, in crate_root_src
                fail("No {} source file found.".format(" or ".join(file_names)), "srcs")
Error in fail: attribute srcs: No lib.rs or bar.rs source file found.
ERROR: Analysis of aspect '@io_bazel_rules_rust//rust:rust.bzl%rust_clippy_aspect of //foo:bar' failed; build aborted: Analysis of target '//foo:bar' failed

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[djmarcin]

@googlebot I signed it!

[google-cla]

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[dfreese]

Thanks!

If you're feeling ambitious, it looks like it would make more sense to pass in the crate_info.type into crate_root_src, instead of file_name, since this is showing up elsewhere:

rules_rust/rust/private/rust.bzl

Line 204 in afee872

root = crate_root_src(ctx.attr, ctx.files.srcs, file_name = "main.rs"),

[djmarcin]

Sure, I'll give that a look. I signed the CLA for my company, but it looks like that takes a few days to get approved, so I guess I've got time. 😂

[djmarcin]

@dfreese Updated to pass type to crate_root_src instead, and the CLA is good to go as well.

[dfreese]

Thanks!