Bugfix recursive rule-naming

[vHeemstra]

Fixes #3

Problem/Bug

Object.assign changes the imported rules in-place. Therefore, on each run, rules already has the additonal scss/ variants.

So it makes scss/ variants for each of these, resulting in recursively added doubles and more. See example:

Run 1:

{
  "some-rule": null,
}

becomes

{
  "some-rule": null,
  "scss/some-rule": null,
}

Run 2:

{
  "some-rule": null,
  "scss/some-rule": null,
}

becomes

{
  "some-rule": null,
  "scss/some-rule": null, // <------- this. ---------,
  "scss/some-rule": null, // --- This overwrites ---`
  "scss/scss/some-rule": null,
}

Run 3:

{
  "some-rule": null,
  "scss/some-rule": null,
  "scss/scss/some-rule": null,
}

becomes

{
  "some-rule": null,
  "scss/some-rule": null, // <------- this. ---------,
  "scss/some-rule": null, // --- This overwrites ---`
  "scss/scss/some-rule": null, // <------- this. ---------,
  "scss/scss/some-rule": null, // --- This overwrites ---`
  "scss/scss/scss/some-rule": null,
}

...etc...