Restore Babel cache invalidation

[novemberborn]

#1798 caused drastic regressions in our ability to invalidate the precompilation cache. This was intentional, as we wanted to upgrade to newer Babel 7 betas.

There's a few TODO comments in lib/babel-pipeline.js with pointers on how to improve our cache invalidation:

ava/lib/babel-pipeline.js

Line 134 in a5de369

// TODO: Check for `partialTestConfig.config` and include a hash of the file

ava/lib/babel-pipeline.js

Line 111 in a5de369

// TODO: Take resolved plugin and preset files and compute package hashes for

With regards to plugins & presets, we should figure out what package the resolved plugin / preset is from, and then use package-hash to compute a hash. If the plugin / preset is a file in the current project (and not a dependency) we should just hash the file. The same if it's from outside of the current project.

We're now resolving Babel options for every file that's being compiled. We should therefore have a lookup of resolved plugins / presets and config files to previously computed hashes.

package-hash has an asynchronous mode which we could use, though we'd have to change the pipeline to be asynchronous itself, so that'd be a bit more work.

userOptions may come from an ava.config.js file (at least when #1761 lands). Consequently they may include non-serializable values. Perhaps we should serialize it using Concordance before taking the hash.