Add sanity check for LIB_DEPENDS and RUN_DEPENDS.

Ports does not support relative paths in ${TYPE}_DEPENDS and actually breaks
the dependency registration with pkgng. This occurs in ACTUAL_PACKAGE_DEPENDS
where ${PORTSDIR} is stripped from the dependencies' directory and uses the
rest of the string as the package origin. pkg(8) then fails to detect package
origins with relative paths.

Approved by: portmgr (bapt)