crypto/aes: move to crypto/internal/fips/aes

The crypto/aes <-> crypto/cipher interfaces and the hardware support
upgrades were layered over the years, and had grown unwieldily.

Before: conditionally wrap the private crypto/aes type in private types
that implement an interface that's interface-upgraded by crypto/cipher
to replace the generic implementation in crypto/cipher.

crypto/aes depended on crypto/cipher, which is backwards.

After: provide concrete exported implementations of modes in
crypto/internal/fips/aes that crypto/cipher returns if the input Block
is the crypto/internal/fips/aes concrete implementation.

crypto/aes and crypto/cipher both depend on crypto/internal/fips/aes.

Also, made everything follow go.dev/wiki/TargetSpecific by only putting
the minimal code necessary and no exported functions in build-tagged
files.

The GCM integration still uses an interface upgrade, because the generic
implementation is complex enough that it was not trivial to duplicate.
This will be fixed in a future CL to make review easier.

For #69536

Change-Id: I21c2b93a498edb31c562b1aca824e21e8457fdff
Reviewed-on: https://go-review.googlesource.com/c/go/+/624395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>

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