Improve import time

[pitrou]

Before:

$ time python -c "import sys, dask; print(len(sys.modules))"
507

real	0m0.480s
user	0m0.428s
sys	0m0.048s

After:

$ time python -c "import sys, dask; print(len(sys.modules))"
192

real	0m0.195s
user	0m0.180s
sys	0m0.008s

[pitrou]

Any objection to this?

[mrocklin]

None from me