fix(forge): same way to flatten for all commands

[grandizzy]

Motivation

• closes forge verify-contract --flatten produces different flattened code than forge flatten command #11754 use consistent flatten for forge flatten, forge verify-contract and cast src

Solution

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[DaniPopes]

in forge flatten we have a fallback for using the old flattener:

foundry/crates/forge/src/cmd/flatten.rs

Line 52 in 1cc7cc9

let flattened = match flattener {

can we unify all the invocations to a single common util function that handles this?

[grandizzy]

in forge flatten we have a fallback for using the old flattener:

foundry/crates/forge/src/cmd/flatten.rs

Line 52 in 1cc7cc9

let flattened = match flattener {

can we unify all the invocations to a single common util function that handles this?

good point, added in d60e7b2 and extended to cast src as well