cache is split into multiple .pack files which represent usage of cache items
this allows to only read/deserialize files that are needed
and reduces the need to rewrite .pack files
while still packaging multiple cache items together
lazy (de)serialization system has changed to allow writeLazy and writeSeparate
add Compiler.intermediateFileSystem
avoid using `path` and `fs` module when possible
move `join`, `mkdirp` and `dirname` into utils
join and dirname is optional in FileSystem interface
remove mkdirp from Filesystem interface
Buffers.concat creates a new buffer. On a large project, when using resolver
cache and pack store, total size of buffer can reach hundreds of megabytes. Use
streaming write instead.