fix(package): Ensure we can package directories ending with '.rs'

[epage]

What does this PR try to resolve?

This likely only affects -Zpackage-workspace but it might have also broken dependencies whose path ends with .rs as
well

This broke in #14961

How should we test and review this PR?

Additional information

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[epage]

Should we do a beta backport?

[weihanglo]

Thanks!

[weihanglo]

Should we do a beta backport?

If we want to be extra cautious, yes, though I cannot find a way to break any stable features.
I'll do the remaining work if you open a beta backport PR.

[epage]

Confirmed: this is visible with stable behavior.

$ cargo new foo --lib
$ mv foo foo.rs
$ cargo add --path foo.rs
$ cargo check
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.03s
$ cargo +nightly check
error: failed to get `foo` as a dependency of package `cargo-15240 v0.1.0 (/home/epage/src/personal/dump/cargo-15240)`

Caused by:
  failed to load source for dependency `foo`

Caused by:
  Unable to update /home/epage/src/personal/dump/cargo-15240/foo.rs

Caused by:
  Single file packages cannot be used as dependencies
$ nargo check
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s

[weihanglo]

Beta backport submitted: #15248