Providers: move all digests

Providers: move all digests

From providers/{common,default,legacy}/ to providers/implementations/
However, providers/common/digests/digest_common.c stays where it is,
because it's support code rather than an implementation.

To better support all kinds of implementations with common code, we
add the library providers/libcommon.a.  Code that ends up in this
library must be FIPS agnostic.

While we're moving things around, though, we move digestscommon.h
from providers/common/include/internal to providers/common/include/prov,
thereby starting on a provider specific include structure, which
follows the line of thoughts of the recent header file reorganization.
We modify the affected '#include "internal/something.h"' to
'#include "prov/something.h"'.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10088)