This change re-introduces skipping AOT snapshot builds if input sources
and outputs have not changed since the last snapshot build, assuming a
build for the same platform in the same build mode.
This reverts commit 3d5afb5a81052b7d55661549320d6d43f893f448.
It includes the following changes relative to the original:
1. Include the entrypoint source in the checksums
2. include the build mode in the checksums
3. include the target platform in the checksums
1ee9400130