Warn when importing files outside of paths.appSrc

[Timer]

Can we display a warning when importing non-node_module files from outside of paths.appSrc?

Files outside of appSrc aren't compiled causing UglifyJS to fail during build (but works fine during development).

[gaearon]

People might be using this for #1107.
Not officially supported but we should address it if we close this loophole.

[gaearon]

Fixed by #2189.