[StreamExecutor] Rename ::perftools::gputools -> ::stream_executor, part 1.

Step 1 of re-namespace'ing StreamExecutor into ::stream_executor.

This moves everything inside of stream_executor/..., and leaves a
namespace alias into ::perftools::gputools.  The next steps will clean
up users to use the new namespace.

This is mostly a mechanical change, but it also includes a bunch of
non-mechanical changes that ideally would be split out into separate
patches.  Unfortunately they all sort of need to be shoved in here for
various reasons:

 - forward declarations need to be in the same namespace as the actual
   types, so we need to change all forward declarations of
   StreamExecutor types in this one patch.

 - Uses of these forward declarations need to be changed to the new
   namespace (or otherwise we need to add a namespace alias to the
   relevant header, but this is pretty ugly).

 - Various initialization code needs to live in StreamExecutor's "real"
   namespace, so all this needs to be changed.

PiperOrigin-RevId: 193256128