exclude_re on external modules does not apply inside module files #633

#[mutants::exclude_re("...")] works on an inline module, but the same
attribute on mod external; does not apply inside external.rs.

// src/lib.rs
#[mutants::exclude_re("replace skipped")]
mod external;

// src/external.rs
pub fn skipped() -> bool {
    true
}

pub fn retained() -> bool {
    true
}

cargo mutants --list still includes the function-value mutant for
external::skipped. Moving the same module body inline excludes it as
expected.

The active patterns are kept while visiting one syntax tree, but are not
carried with an external-module reference into the referenced file. Module
scope should be independent of whether the module is inline or stored in
another file, including across nested external modules.