packages.bbclass: Correct the check for conflicts with renamed packages

The original solution replaced all overrides with the package name that
was being checked. This can have unforseen consequences where some
variable involved in defining the value for the PKG:<package> variable
may rely on some override which is not set as expected. It also meant
that any PKG variable set using an override would not be caught, e.g.,
PKG:${PN}:${MACHINE} = "${PN}-dev" (made up example that would always
fail with the old code).

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>