For consistency with the other named types in this package, this
change renames the unexported rsaPublicKey struct to pkcs1PublicKey
and positions the declaration up with the other similarly-named
types in pkcs1.go.

See the final comment of #19355 for discussion.

Change-Id: I1fa0366a8efa01602b81bc69287ef747abce84f5
Reviewed-on: https://go-review.googlesource.com/37885Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>