crypto/x509: rename and reposition rsaPublicKey struct declaration

crypto/x509: rename and reposition rsaPublicKey struct declaration

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/37885
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>