mosquitto: fix provides <=> package name conflicts

mosquitto: fix provides <=> package name conflicts

The virtual package declared by PROVIDES must not have the same name as the
variant declaring it, otherwise buildroot will fail with errors like:

    cp: '.../pkginfo/mosquitto.provides' and '.../pkginfo/mosquitto.provides' are the same file

In order to fix the above error, rename the existing "mosquitto" and
"libmosquitto" packages into "mosquitto-ssl" and "libmosquitto-ssl"
respectively.

Also substitute use of $(PKG_NAME) with literal "mosquitto" in
Package/* defines to improve readability of the Makefile.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>