CLN: import packers/gbq/html only on-demand (GH9482)

[jreback]

closes #9482
so a 30% reduction in import time

[jreback-~/pandas] time python -c 'import pandas' 0.788u 0.120s 0:00.91 98.9% 0+0k 0+7io 0pf+0w [jreback-~/pandas] time python -c 'import pandas' 0.782u 0.115s 0:00.91 97.8% 0+0k 0+0io 0pf+0w [jreback-~/pandas] time python -c 'import pandas' 0.790u 0.121s 0:00.91 100.0% 0+0k 0+0io 0pf+0w 

[jreback-~/pandas] time python -c 'import pandas' 0.270u 0.091s 0:00.37 97.2% 0+0k 10+20io 0pf+0w [jreback-~/pandas] time python -c 'import pandas' 0.262u 0.087s 0:00.35 97.1% 0+0k 0+0io 0pf+0w [jreback-~/pandas] time python -c 'import pandas' 0.264u 0.088s 0:00.36 94.4% 0+0k 0+0io 0pf+0w 

[jreback]

This seems to work (meaning that it doesn't import these except on-demand), can people give this a try

@TomAugspurger @shoyer @jorisvandenbossche @cpcloud
cc @rockg

from the related issues
cc @thomasj02
cc @mpenning
cc @ndc33
cc @steve3141

[jreback]

any comments on this?

[jorisvandenbossche]

Looks good to me! (gave it a quick test and glance over the code)