build/pkgs/rpy2: use SAGE_PYTHON_PACKAGE_CHECK in spkg-configure.m4

Our system package lists are generated with a special case that looks
for SAGE_PYTHON_PACKAGE_CHECK when spkg-configure.m4 is present. The
rpy2 package already had an spkg-configure.m4 relating to --disable-r,
but its spkg-configure.m4 did not use SAGE_PYTHON_PACKAGE_CHECK. As a
result, it was erroneously included in the package list.

We now use SAGE_PYTHON_PACKAGE_CHECK for the usual "check" phase of
SAGE_SPKG_CONFIGURE, which both allows the user to use rpy2 from the
system and fixes the package list.

build/pkgs/rpy2/spkg-configure.m4

@@ -1,6 +1,6 @@
 SAGE_SPKG_CONFIGURE([rpy2], [
-    sage_spkg_install_rpy2=yes
-  ], [dnl REQUIRED-CHECK
+    SAGE_PYTHON_PACKAGE_CHECK([rpy2])
+], [dnl REQUIRED-CHECK
     AC_REQUIRE([SAGE_SPKG_CONFIGURE_R])
     dnl rpy2 is only needed when there is a usable system R
     AS_VAR_IF([sage_spkg_install_r], [yes], [dnl