build-support: Added makeSymlinks function

I have refractored things a little bit. We need a function that would symlink
a bunch of files created with writeText to a folder. What `environemnt.etc`
uses looks like a fit, because it has really nice syntax where you can specify
`source`, `target` and `mode` so i generalize its functionality.

Because i don't know what is the best way to generalize this please advise or
propose a better soution. I need this to make a bunch of nixos modules
more pure.