Get list of CPT posts in *current* post’s taxonomy term

You can get the Packages of the current Proposal using wp_get_object_terms() and then pass some/all of those into a WP_Query.

$terms = wp_get_object_terms( $post_id, 'packages', array( 'fields' => 'ids' ) );
$args = array(
    'post_type' => 'proposals',
    'tax_query' => array(
        array(
            'taxonomy' => 'packages',
            'terms' => $terms
        )
    ),
);
$related = new WP_Query( $args );

If the current Proposal has multiple Packages, this will return a list of Proposals in any of those Packages. You can add 'operator' => 'AND' to the WP_Query $args to restrict it to only Proposals in all the same Packages.