archive: use a templater to build the metadata file
There are no visible changes here.
I'm starting to wonder if adding the '+' to the 'node' line instead of a
separate key line in 3047167733dc was the right thing to do. The '{node}'
keyword never includes '+' elsewhere, and the way setup.py works, it would
truncate it anyway. Additionally, the file is missing '{p2node}' when 'wdir()'
merges are archived. I thought about adding an 'identify' line that would
correspond to `hg id -n`. But the other nodes are the full 40 characters, and
the output most useful for versioning is the short form. All of this cries out
for customization via templating. (Although maybe having the short identify
line by default is still a good idea.)