pyo3-build-config: make interpreter config and references types public

PyOxidizer will want to create interpreter config files. Rather
than reinvent the logic for reading/writing these files, I think
it makes sense to consume the `pyo3-build-config` crate so we can
use the `InterpreterConfig` type directly. But the symbol needs
to be public to allow us to do that. And in order to allow direct
construction, we need to make all the types referenced by its fields
public as well.

pyo3-build-config/src/lib.rs

@@ -13,11 +13,7 @@ use std::io::Cursor;
 
 use once_cell::sync::OnceCell;
 
-use impl_::InterpreterConfig;
-
-// Used in `pyo3-macros-backend`; may expose this in a future release.
-#[doc(hidden)]
-pub use impl_::PythonVersion;
+pub use impl_::{BuildFlag, BuildFlags, InterpreterConfig, PythonImplementation, PythonVersion};
 
 /// Adds all the [`#[cfg]` flags](index.html) to the current compilation.
 ///