Make internal pprof packages available to cmd/trace.
cmd/trace needs access to them to generate symbolized
svg profiles (create and serialize Profile struct).
And potentially generate svg programmatically instead
of invoking go tool pprof.

Change-Id: Iafd0c87ffdd4ddc081093be0b39761f19507907a
Reviewed-on: https://go-review.googlesource.com/21870
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>