Add unstable foo::bar extern command line arguments

Also refactors some of the crate name parsing code and adds unit tests

Issue #122349

Co-authored-by: León Orell Valerian Liehr <me@fmease.dev>

compiler/rustc_session/src/options.rs

@@ -2331,6 +2331,8 @@ options! {
         "the size at which the `large_assignments` lint starts to be emitted"),
     mutable_noalias: bool = (true, parse_bool, [TRACKED],
         "emit noalias metadata for mutable references (default: yes)"),
+    namespaced_crates: bool = (false, parse_bool, [TRACKED],
+        "allow crates to be namespaced by other crates (default: no)"),
     next_solver: NextSolverConfig = (NextSolverConfig::default(), parse_next_solver_config, [TRACKED],
         "enable and configure the next generation trait solver used by rustc"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],