initial checkpoint for "buildArchitectures"

this is a string like "browser+os"
some other stuff that goes together
fix for removing fields from Builds
oops! actually use specific arch!
fix wrong arch in plugin package search

the "We pass" comment was stolen back from devel

basically, whenever doing eachUsedBuild, you really want to pass in the
arch of the overall compilation (or for plugins, the host arch), not the
arch of the unibuild you happen to be sitting in right now.  because if
you're sitting in a generic "os" unibuild, it won't know whether to use
os.mac or os.linux or whatever if you pass in "os"!