Fix sass import edge case

[jondlm]

↪️ Pull Request

Fixes #7163.

Some packages, such as @primer/css, specify a sass property in their package.json that is a string. Parcel was throwing an error TypeError: Cannot create property 'importer' on string before this fix.

🚨 Test instructions

Test case included in the PR.

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[mischnic]

I think a better test case would be

    await fsFixture(overlayFS, dir)`
      index.js:
        import './main.css';

      main.css:
        @import './edge/main.scss'

      edge
        package.json:
          {
            "name": "edge",
            "sass": "main.scss"
          }

        main.scss:
          .foo {
            .bar {
              color: green;
            }
          }
        `;

because I'm actually surprised that the Sass transformer loads configs inside node_modules. Babel, PostCSS, PostHTML don't do this.