Improve special character handling in package revisions

[frantisekhanzlikbl]

Currently, the yarn plugin generates invalid nix paths when package revisions with paths (i.e. workspace: and patch: protocols) contain nix store illegal characters. This is really annoying when patching scoped packages, as the @ symbol is also forbidden in store paths.

Generated code without this patch:

{
  src = ./.yarn/patches/@locator-babel-jsx-npm-0.2.4-2784012346.patch;
  #                     ^ syntax error
}

Generated code with this patch:

{
  src = builtins.path { name = "__locator-babel-jsx-npm-0.2.4-2784012346.patch"; path = ./${"./.yarn/patches/@locator-babel-jsx-npm-0.2.4-2784012346.patch"}; };
}

Note 1: Only store-illegal paths are changed. All valid paths are still generated the normal way. I have verified that the generated manifest in test/ is exactly the same with this patch.

Note 2: I tried to follow the formatting used elsewhere in this file, but I'm not sure about, for example, the preferred line lengths.

Note 3: I think there is nothing inherently non-portable about the code, but I don't have a darwin machine to test it on, so verifying that everything works would be much appreciated. :)