versioner: use a virtual filesystem for input files.

Use an InMemoryFileSystem to store and share input files across
compilations.

This improves the result of `time versioner` further, from:
    versioner  109.12s user 17.43s system 2433% cpu 5.201 total
to:
    versioner  112.20s user 1.38s system 2416% cpu 4.700 total

Bug: http://b/32748936
Test: python run_tests.py
Change-Id: I72d37b7c30850b8399cc40338247700fe3e7b2f9