Assets added via a plugin are not source files

Fixes #2488.

Assets added via "no handler" already have their source files added to
`sources` via the `sources.push(relPath)` in the sourceItems loop, and
assets added via a handler should not appear as source files themselves
--- the file that triggered the handler should!

This fixes packages like `mizzao:build-fetcher` which add static
assets via a plugin.