Prefer longest match in path dependency search

[dtolnay]

The previous behavior was the match having the alphanumerically least name would be used in the replacement, which isn't a justifiable behavior.

[dependencies]
a = { path = "../b/a" }  # $A/src/lib.rs
b = { path = "../b" }    # $B/src/lib.rs
c = { path = "../b/c" }  # $B/c/src/lib.rs

After this PR the new behavior uses the longest match: $A/src/lib.rs, $B/src/lib.rs, $C/src/lib.rs.

Always using shortest match would also produce reasonable behavior. $B/a/src/lib.rs, $B/src/lib.rs, $B/c/src/lib.rs