support `x fmt` for sub and outside of rust directories This PR makes `x fmt` to be succesfully work from sub or outside dir of rust. The following cases are tested and the results were successful. ```sh # from rust/library directory ../x fmt compiler --check ../x fmt compiler/rustc # from outside of rust directory ../../rust/x fmt library/std library/core --check ../../rust/x fmt library/std ``` Resolves #108004 r? ``@albertlarsan68``