Fix Python bindings build order and installation for OSS

Summary:
These changes fix the folly Python bindings build in OSS environments, specifically to support building thrift-python (the modern Python bindings for Apache Thrift).

**Background**: As part of making thrift-python available in OSS (T235311085), we discovered that the folly Python build had several issues that prevented successful builds outside of Meta's internal Buck environment.

**Key fixes**:

1. **Build order**: Separated Cython API header generation from linking. Previously, `setup.py build_ext` would try to link before `folly_python_cpp` was built, causing undefined symbol errors. Now:
   - `--api-only` mode generates just the API headers (iobuf_api.h)
   - C++ library (folly_python_cpp) builds against these headers
   - Python extensions link against the built C++ library

2. **Library path**: Added CMAKE_CURRENT_BINARY_DIR to library search path so the linker can find folly_python_cpp.so in the build directory.

3. **Installation**: Switched from deprecated `setup.py install` to `pip install` on the built wheel. The old method created eggs (zip archives) that don't work in custom install directories without .pth file support.

4. **Package structure**: Creates `__init__.py` for the folly package and installs .pxd files for downstream Cython code (thrift uses `from folly.iobuf cimport IOBuf`).

5. **Dependencies**: Added libaio to Linux dependencies (required by folly).

Reviewed By: vitaut

Differential Revision: D90277822

fbshipit-source-id: 344dd256cf42e96a1a2b943d9f9cb7a059a35238

Author: Satish Kumar

build/fbcode_builder/manifests/folly

@@ -45,6 +45,7 @@ openssl
 zlib
 
 [dependencies.os=linux]
+libaio
 libiberty
 libunwind