Plugin-level scripts install via symlink back into the source tree

[ckreibich]

I think this is an old optimization aimed to simplify life for developers, but nowadays it means we create a symlink back into zkg's internal state, which is risky and complicates/prevents things like zeek/package-manager#140.

An example:

$ pwd
/home/christian/inst/opt/zeek/lib64/zeek/plugins/packages/zeek-community-id
$ ls -la scripts
lrwxrwxrwx. 1 christian christian 82 Oct 20 14:46 scripts -> /home/christian/inst/opt/zeek/var/lib/zkg/clones/package/zeek-community-id/scripts/