fix(flatten): sort by loc path and loc start

[grandizzy]

ref foundry-rs/foundry#11417
for

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.19;
...

function convert(UD60x18 x) pure returns (uint256 result) {
...
}

function convert(uint256 x) pure returns (UD60x18 result) {
...
}

we have items as

[
  (8378, ItemLocation { path: "lib/prb-math/src/ud60x18/Conversions.sol", start: 462, end: 469 }), 
  (8409, ItemLocation { path: "lib/prb-math/src/ud60x18/Conversions.sol", start: 833, end: 840 })
]

but we sort only by path, hence order could be reversed.

Sort also by loc.start to make sure order is preserved

[mattsse]

ah, makes sense