Add support for split targets of mains for definitions

[davidje13]

Currently multiple mains entries are supported, but they are always bundled into a single TypeScript "program" and compiled into the output directory.

Configurations such as the following do not work as expected, because the folder structure changes during compilation:

    mains: {
      index: 'index',
      page1: 'pages/page1',
    },

Need to find out if TypeScript has any support for this type of functionality (simply creating multiple programs is not ideal since it will result in duplication of common dependencies, rather than sharing dependencies by changing relative imports)