Correct the relative path for traceur-loaded nodejs modules.

Using Module._compile() causes evaled code to require() relative to the
caller of _compile(). We want the require() to be relative to the file containing
the es6 code we transcoded.

Here we include a file-local function `require()` during transcoding of modules.
The local require() calls a runtime function with the module name and the required path.
The runtime function resolves the required path iff its relative.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1434

Closes #1434.