Improve caching behavior for tasks

[scalacenter]

Improve caching behavior for tasks

[nigredo-tori]

@julienrf, scalaJSNativeLibraries only helps with *.js files - and webpack supports a lot more (this also reuses pre-existing key from another library, which is probably not a good idea). Webpack gives a lot of flexibility in collecting and mapping files, and we shouldn't put restrictions on it. At least not without an escape hatch.
A separate key for the monitored files would be a convenient and flexible solution, in my opinion, though it would place a burden on API user to collect the files (which doesn't look like an insurmountable problem). An alternative would be some combination of directory list with filtering, I suppose - something like unmanagedSourceDirectories/unmanagedSources/includeFilter/excludeFilter.

By "generated data" I mean the generated json string. Though simply concatenating all the relevant input to writePackageJson, and using it as a "hash" seems to do the trick here - and is extremely simple to implement. No need to prematurely optimize/generalize this at the moment.

[julienrf]

For the webpack task, I would not disable caching because bundling times can be quite long. An improvement could be to take into account the scalaJSNativeLibraries for the hash. We could also introduce a monitoredResources key, similar to the one introduced by sbt-web-scalajs, but I wonder if there isn’t a more standard solution.

For package.json, indeed the caching mechanism used is wrong. What do you mean by “the generated data”?

[julienrf]

Thanks for detailed explanation. I think we should go with a key for the monitored files.

[dispalt]

This would be very useful for me too.