[ng] dist: vars DIST_TARGETS and DIST_ARCHIVES are no more public

[ng] dist: vars DIST_TARGETS and DIST_ARCHIVES are no more public

Turn them into two internal variables.  This will clean up the
interface a bit, and make the new planned dist-related APIs (still
to be implemented) simpler to use.

* lib/am/distcheck.mk (DIST_TARGETS, DIST_ARCHIVES): Rename ...
(am.dist.default-targets, am.dist.default-archives): ... like
these, respectively.
(distcheck, dist, dist-all): Adjust.
* t/dist-formats.tap (nogzip): Likewise.
* NG-NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>