Either not include source map, or include source in dist

[vidartf]

Currently, building against RC services gives a bunch of webpack build warnings:

WARNING in C:/dev/nbdime/node_modules/@jupyterlab/services/lib/builder/index.js
Module Warning (from C:/dev/nbdime/node_modules/source-map-loader/index.js):
(Emitted value instead of an instance of Error) Cannot find source file '../../src/builder/index.ts': Error: Can't resolve '../../src/builder/index.ts' in '[...]\nbdime\node_modules\@jupyterlab\services\lib\builder'