Can verify somewhat relative paths: ./test from \hphp

Can verify somewhat relative paths: ./test from \hphp

Summary:
Previously verify failed on 753 files when run on `./test`:
```
~/fbsource/fbcode/hphp]$ buck2 run hack/src/hackc:hackc -- verify -l -a ./test
```
(the problem is using the `./` in the path rather than just `test`)
Because the assembler couldn't tokenize `HHAS` that looks like this:
```
.includes {
  ./test/slow/thrift/common.inc
}
```
So here we modify the closure that assembles `includes`, add an `expect_decl_into_ffi_str` method, and modify the decl regex.

Reviewed By: aorenste

Differential Revision: D38135736

fbshipit-source-id: 92321544c8fe2d9e752f11a9ac4468a8c4b431f1