Improve bundle reuse/cache for unchanged imports

[skeggse]

If I change a file, but don't modify the imports, I'd like rollup to avoid rebuilding all the imported modules as well. I realize this might not always be possible, but it would speed up builds, which even when aggressively cached take half a minute.

EDIT: I may be misinformed - it looks like this should already be happening.

[TrySound]

@skeggse Yep. We already introduced cache option and trying now to improve some things. Ref #723