package manager should not rewrite tpl files in place, but copy to a new staging area

[jurgenvinju]

Describe the bug

The packager is a rewriter of project locs to lib locs for the sake of browsing through library files in the IDE (for example while debugging. It is a part of the rascal-maven-plugin.

The packager reads tpl files and replaces them on disk with new tpl files. Because of this, if the packager has run once, it will run again on different input files (which have already been rewritten). It could lead to very subtle bugs. At the moment it leads to jar packages for which it is unclear how many times the packager has tried to rewrite the .tpl files.