xgettext: Rust: Recognize 'gettextrs::gettext' invocations like 'gettext'

xgettext: Rust: Recognize 'gettextrs::gettext' invocations like 'gettext'
invocations.

Reported by Schmiddiii <gnu-savannah@schmidhuberj.de>
at <https://savannah.gnu.org/bugs/?67601>.

* gettext-tools/src/x-rust.c (ts_symbol_scoped_identifier): New variable.
(extract_from_function_call): Accept a scoped_identifier as first argument.
(extract_from_node): Allow a scoped_identifier as callee.
(extract_rust): Initialize ts_symbol_scoped_identifier.
* gettext-tools/tests/xgettext-rust-1: Add another test case.
* gettext-tools/tests/xgettext-rust-6: Likewise.
* NEWS: Mention the fix.