* Fixes that `ng_package` does not work generate UMD bundles on Windows because the `esm5/` files are not written to the output directory. This is because `rootDirs` and `rootDir` are posix paths and cause invalid relative paths when mixed with Windows backslash paths.

* Currently when building the ES5 and ES2015 output, `ngc_wrapped` will fail because it tries to write the `fs.openSync` the tsickle output file at the same time. This causes a runtime exception in Windows and can be fixed by just writing the externs for ES5 mode to the proper ES5 "output root".

Currently when building the `ng_package` multiple times, the old `ng_package` output will be copied over to the new `ng_package` content. Resulting in packages like `src/cdk/npm_package/npm_package/npm_package/AND_MORE`.

This happens because currently all TypeScript definition files are resolved from within the `binDir`. This is just wrong because it could then take up the `d.ts` files from the previous `ng_package` output. All typescript definitions that belong to the target package, should be resolved through Bazel and copied based on that computation.

Also fixes that `esm` files aren't written to the `ng_package` on Windows. This is because we try to flatten paths using the `path.delimiter` while the path is always using Posix delimiters (causing the paths to be incorrect)

…h multiple input files

* Currently when building a `ng_module` with Bazel and having the flat module id option set, the flat module files are not being generated because `@angular/compiler-cli` does not properly determine the entry-point file.

Note that this logic is not necessarily specific to Bazel and the same problem can happen without Bazel if multiple TypeScript input files are specified while the `flatModuleIndex` option has been enabled.