Even if we don't know the exact method being called, include it
in the call graph with the is_prototype flag. In particular, we
can still make use of return types from prototype methods, as
PHP 8 makes LSP violations a hard error.
Most other places are adjusted to skip calls with !is_prototype.
Maybe some of them would be fine, but ignoring them is conservative.
Even if an explicit return type is given, we might still infer
a more narrow one based on return statements. We shouldn't
pessimize this just because a type has been declared.
This only moves the files, adjusts the build system, exports APIs
and does minor fixups to make sure the code builds.
This does not yet try to make the optimizer usable independently
of opcache.
Closes GH-6642.
