Capture popularity for packages used via `pub global activate`

[kevmoo]

Tracking issue for back-end analysis – currently we don't capture popularity for packages that are used via pub global activate.

grinder, stagehand, aqueduct, coverage are all "popular" via this usage, but this is not tracked anywhere.

[kwalrath]

Also webdev.

[sigurdm]

Not sure exactly where we want to take this.

Maybe we can recognize "global activatable packages" (by seeing which packages have files in bin/) and use that to rank these packages relatively based on their download count.