[seaborn-] skip slow imports on every plot after the first plot

[midichef]

This PR speeds up plots using Seaborn.

The imports of matplotlib.pyplot and seaborn were being done fresh on every call to plot*-ext, instead of just the first call. On my system that added a delay of about 2 seconds each time. After this PR the delay only happens on the first call.

[saulpw]

vd.importExternal should be caching the package, just like how import does, so subsequent re-imports should take no time at all. Rather than manually caching it, let's fix vd.importExternal properly.

[midichef]

importExternal currently is caching the packages, but it's doing so inside the spawned process, where ext_plot_seaborn is run with multiprocessing. The next spawned Seaborn process can't see those cached packages.

[saulpw]

I see, sorry for not reading more carefully. Thanks for submitting this!