Implement a module import scheme that relies upon absolute paths for external typedefs

[drewbryant]

Current issue is that the source code needs to be compiled in multiple locations, each of which must have the correct number of parent directories to the externs/ts typedefs (because only relative paths are permitted). One workaround would be to symlink externs/ts to each build location and then change all import references to account for this.

All of this trickery would be unnecessary if TypeScript supported a requirejs-style path config specification, but it does not at the moment.

This module path config feature is being actively discussed within the TypeScript community, so
opting to see how it plays out before implementing more complex work-arounds.

For discussion, see: microsoft/TypeScript#293

Maintaining this issue as a reminder to periodically check-in on the progress of the TypeScript path config specification.