Source vs require

[odino]

Now require is not an alias of source aymore.

source, like in bash, imports a script and gives it full
access to the global environment.

require instead imports the script, but does not allow it
to read or set variables at the global level. If a required
scripts needs to "pass" variables to the caller, it can simply
return them and they will be available to th caller.

Example -- say this is our module, ip-finder.abs:

return f() {
  return `curl icanhazip.com`
}

Then we can simply use it like this:

ip_finder = require("ip-finder.abs")

echo("My IP is %s", ip_finder())

I have also made an important change while we wait for modules and the ABS package manager to be implemented (cc @mingwho): I've made require(...) use paths that are relative to the current script, just like in NodeJS etc.

To illustrate the problem:

/A
  /dir/B
  /dir/C

Say that A imports B, and B imports C. If you use source, B will have to use source("./dir/C") as source paths are always relative to the initial execution context.
With require B instead can require("C") since the import path is always relative to the current script. When @mingwho works on the installer, we'll have to tweak how we locate the required script making sure we can include installed modules.

With these changes, I've also gotten rid of the globalEnv variable which is a global we use to access the main ABS execution environment. I replaced it making sure that the env is being passed around as needed.