base: check for existing prefix when expanding names in PACKAGECONFIG

base: check for existing prefix when expanding names in PACKAGECONFIG

When the DEPENDS are added as part of the PACKAGECONFIG logic the list of
packages are expanded so that any required nativesdk-/-native/multilib prefixes
and suffixes are added.

However the special handling of virtual/foo names doesn't check that the prefix
already exists, which breaks under nativesdk as in that situation there's an
explicit nativesdk- prefix *and* MLPREFIX is set to nativesdk-.  This results in
the same prefix being applied twice, and virtual packages such as virtual/libx11
ending up as virtual/nativesdk-nativesdk-libx11.

(From OE-Core rev: 55ca1fb8f0e81ff739b3c46897e43356d1f760c3)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>