to keep track of runtimes, used exports depending on runtime
modules can have different hashes and generated code depending on runtime
chunks are only considered as equal for MergeDuplicateChunks and SplitChunks when exports used in contained modules are equal
added `optimization.usedExports: "global"` to opt-out from used exports analysis per runtime
added `splitChunks.usedExports` and `splitChunks.cacheGroups[x].usedExports` to enable/disable used exports comparing for modules
`splitChunks.usedExports` defaults to `true` in production
fix a bug with prefetching initial chunks
InitFragments have get[End]Content methods which get a sourceContext
RuntimeModules have implicit chunk and compilation
changed default to `output.ecmaVersion: 2015`
Tobias Koppers
evilebottnawi