Upgrade/install: Allow plugin and theme updates from a uploaded .zip file.

Props mariovalney, cyberhobo, imath, shaunandrews, mariovalney, earnjam, desrosj, dd32, folletto, swissspidy, melchoyce, pento, joshuawold, psykro, clorith, ahortin, galbaras, pingram3541, joyously, doobeedoo, karmatosed, poena, whyisjake, earnjam, sergeybiryukov, audrasjb, azaozz.

Fixes #9757.
Built from https://develop.svn.wordpress.org/trunk@48390


git-svn-id: https://core.svn.wordpress.org/trunk@48159 1a063a9b-81f0-0310-95a4-ce76da25c4cd