[native_assets_builder] Link hooks output directory with different entry points

[dcharkes]

Once we add support for tree-shaking (#153), it's possible that a user compiles for two different entry points.

In such case, we should share the build hook outputs, but the link hook outputs should not share their output directory.

In the following scenario we'd like the 3rd build to cache the link hooks:

1. dart build bin/entry1.dart (runs build hooks, and link hooks)
2. dart build bin/entry2.dart (runs link hooks)
3. dart build bin/entry1.dart (uses the cached link hook result from invocation 1.)

[dcharkes]

(Somewhat related to #39 (comment), which is also about not wanting unrelated invocations unintentionally invalidating the cache.)