zpm: refactor long-ish line

This line will grow longer when zar is replaced with zapp.
Use "zapp" instead of "zar"

Compared with "zar", "zapp" is easier to pronounce and sounds better.
It is short for "ZeroVM Application". We might reuse "zar" again in
the future for a more generic "ZeroVM Archive" -- archives that cannot
be executed directly, but which contain libraries or data.
zapp.yaml: use "zapp", not "ZAP"

We should always use the double-p version.
Lowercase "zapp" everywhere

If we want to make "zapp" a word like "package" or "module", we should
write it in lowercase everywhere. That will make the term seem like an
ordinary word.
Merge pull request #87 from mgeisler/zapp

Use zapp instead of zar everywhere