stream: move prefixed files into internal/streams.
This simplifies the lib/ folder, and hides _stream_readable.js and peers into lib/internal/streams. All the tests have been updated to use the main stream module, rather than requiring from the internals.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows [commit guidelines][]
Affected core subsystem(s)
stream