Add support for stream wrapper in include_path.

Summary: set_include_path now supports parsing of stream wrapper path (e.g. proto://path).

Passing vfsstream knownFileCanBeResolved test.
Closes https://github.com/facebook/hhvm/pull/4237

Reviewed By: @paulbiss

Differential Revision: D1678642

Signature: t1:1678642:1416443955:d08b42d8c76ec020422e9458e996c3bfacbde391
Fix build with ocamlbuild 3.12

Summary: See comment. We should discuss dropping support for 3.12
tomorrow, I'm sick of this shit.

showercry

saddoctor

Reviewed By: @int3

Differential Revision: D1692564

Signature: t1:1692564:1416443100:f967e8be2d1442d12c06a194fb54ff71a8322517