`rpy2`: Document that it is a "semistandard" package

[mkoeppe]

... in the SPKG.rst of r and rpy2.

Also, for dummy packages such as r, do not show the message "If the system package is installed, ./configure will
check whether it can be used."

see https://groups.google.com/g/sage-devel/c/0Rl6YbnPNmM

Also use the feature argument in the lazy_import of rpy2 in sage.interfaces.r, so that system package info will be displayed when rpy2 is not installed.