Add support for absolute path to babel plugin

[ChristopherBiscardi]

When using Snowpack with a custom output directory like: snowpack --dest public/web_modules, the babel plugin can't use a path that looks like public/web_modules/import-map.json (like in this example) because you end up with ./web_modules/public/web_modules/import-map.json.

const path = require("path");
module.exports = {
  plugins: [
    [
      require.resolve("./forked-snowpack.js"),
      {
        importMap: "public/web_modules/import-map.json"
      }
    ]
  ]
};

This is because dir is automatically added to the path to require the import map in the plugin, even though the path to the import map isn't related to the web server web_modules path location at all (as far as I can tell).

This PR adds a backwards-compatible solution that lets people use an absolute path instead of a relative path. I think the better solution is to not use dir at all to construct the import path, but to remove that would be a breaking change so I opted for this solution, which is enough for my needs.

If this is useful and a good approach for y'all I can add test fixtures to the PR as well.

[FredKSchott]

Makes sense! Would love an added test just to make sure that we don't regress in the future

[ChristopherBiscardi]

For the tests I had to add an additional call to babelPluginTester because the library doesn't support an options.js, only options.json, which is needed to get the absolute path to the import map.

[FredKSchott]

LGTM, fixtures solution makes sense, and we'll keep an eye on it going forward to make sure it doesn't get too messy