Treat RequireJS dep paths as relative to current file

This PR changes the requirejs plugin to treat dependency paths as relative to the current file.

The previous behavior was as follows. Suppose we have `foo.js` and `mydir/bar.js`. If `mydir/bar.js` required `../foo.js`, then the requirejs would try to open `path.join(server.options.projectDir, "../foo.js")`. With this PR, it finds the correct `foo.js`.

Includes a passing test case.

Example of relative RequireJS dep paths: https://github.com/jquery/jquery/blob/6bff3bf7d72383c1458004d4aae454b61caa400f/src/attributes/classes.js#L2