etc/xbps-src.conf: change XBPS_{DIST,MASTER}DIR default paths.

From now on XBPS_DISTDIR (aka xbps-packages) will be stored into
XBPS_MASTERDIR, this way there's no need to make a bind mount of it.
Deprecate XBPS_DISTDIR and make XBPS_MASTERDIR/xbps-packages mandatory.

This changeset deprecates XBPS_DISTDIR completely, and instead
the "xbps-packages" repository must always live in XBPS_MASTERDIR.

Why? because this avoids an additional bind mount of it and makes
xbps-src usage simpler than ever.

-D flag has been kept for compatibility, but it's a NOP.