libxbps: fixed regression introduced in 0.58.

While looking for dependencies, we need to check
if xbps_rpool_get_pkg() returned a suitable match;
and then validate its result.

This fixes the update_and_install test case that
was reverted via #218.

tests/xbps/libxbps/shell/install_test.sh

@@ -598,7 +598,6 @@ atf_test_case update_and_install
 
 update_and_install_head() {
 	atf_set "descr" "Tests for pkg install: update installed version and install new from other repo"
-	atf_expect_fail "fix introduced a regression"
 }
 
 update_and_install_body() {