cmd/pprof: move cmd/internal/pprof back to cmd/pprof/internal

cmd/pprof: move cmd/internal/pprof back to cmd/pprof/internal

CL 21870 moved the entire cmd/pprof/internal directory to cmd/internal/pprof
for use by cmd/trace, but really cmd/trace only needed cmd/pprof/internal/profile,
which became cmd/internal/pprof/profile, and then internal/pprof/profile.

Move the rest back under cmd/pprof so that it is clear that no other code
is reaching into the guts of cmd/pprof. Just like functions should not be
exported unless necessary, internals should not be made visible to more
code than necessary.

Raúl Silvera noted after the commit of CL 21870 that only the profile package
should have moved, but there was no followup fix (until now).

Change-Id: I603f4dcb0616df1e5d5eb7372e6fccda57e05079
Reviewed-on: https://go-review.googlesource.com/32453
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>