profile_imports not in bzrlib

[Martin Pool]

The profile_imports module isn't inside bzrlib, and so isn't installed
by setup.py.  This means that if you use --profile-imports on an
installed copy of bzr it fails.

So we can either 
1 - put it inside bzrlib
2- install it as a top level module
3- or just live with the fact that it fails

I guess 1 has the drawback that bzrlib/__init__ will be exempted from
profiling but I think i'd still prefer that.

-- 
Martin