Summary:
Replace Boost external dependency declarations in the paired `fbcode/folly/io/BUCK` and `xplat/folly/io/BUCK` files with canonical `third-party/boost` labels. Regenerate the paired CMake output and enable the required Boost.Regex alias on Conda platforms.

high risk of breaking target builds #buildall

Reviewed By: dtolnay

Differential Revision: D115600621

fbshipit-source-id: 2085f1876b916e47036cf87e5f59fd2d5a499174