Move digests to providers

Move digest code into the relevant providers (fips, default, legacy).
The headers are temporarily moved to be internal, and will be moved
into providers after all external references are resolved. The deprecated
digest code can not be removed until EVP_PKEY (signing) is supported by
providers. EVP_MD data can also not yet be cleaned up for the same reasons.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8763)