Fixes #24012.
In the case of multiple packages providing a binary with a same name, we
were basically leaving the results undefined (since we set up things in
parallel, and whichever got set up first won). In addition, we were
warning about these cases, even though it's a situation that's expected
to occur.
Instead, in the case of a collision in the binary names, we prefer the
binary provided by the package with the least depth in the dependency
tree.
While I was at it, I also took moved more code to `bin_entries.rs` since
it was starting to get a bit cluttered.
