packagefs: Use '~' as pre-release separator

"[...]" turns out to be not so good an idea after all. In the shell it
would require escaping.
BPackage{Info,Version}: Switch to '~' as pre-release separator

Also allow '.' in the pre-release string (as in "alpha4.1") and in
package and resolvable names.
package-infos in tree: use '~' as pre-release separator