Remove build & src parts from BP plugin basename : the return

Untill / if a better solution is found, removing the build & src parts from the plugin basename will meanwhile make sure the root blog id is correctly set in multisite configs.

See #5652

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8436 cdf35c40-ae34-48e0-9cc9-0c9da1808c22