fix(oss): fb303_thrift -> fb303_core

[ben--]

This diff aligns the OSS target naming with that in the internal BUCK build, enabling fb303 to build with PYTHON_EXTENSIONS enabled as well as clearing the way for other projects which depend on this one.

Additionally, this change enables the "py3" thrift generator to run, which is required by other Meta OSS projects.

Test: This change has been tested by running a full build of EdenFS in the sapling project, pinned to this revision. (There are other changes to folly, fbthrift, rust-common, as well as sapling necessary to get a green build, but this a step on that path.)

[ben--]

This PR depends on an upcoming PR to fbthrift, hence the build error. I'll add a link here once that one is created.

Apologies for submitting in the wrong order.

[ben--]

This PR depends on fbthrift PR 671 which adds py3 support to add_fbthrift_library()