Make Assets API usable from unipackage; use it for dev-bundle-fetcher.

This involves saving the source directory (not just the bundle location) for
static assets so that they can be loaded when unipackages are run.
Don't allow .. in Assets inputs
Include files with no extension handlers as server assets.

Also some miscellaneous test cleanup.
DevBundleFetcher plugin no longer necessary
Merge branch 'assets-unipackage' into linker