Disallow unnamed package on the bootclasspath.

There are currently no classes on the bootclasspath that live in the
unnamed package (empty package name). This CL explicitly forbids it.
This has the side effect of guarding against some classes of bugs,
for example R8 has functionality to generate some helper classes in
the unnamed package that should not be on the to bootclasspath
because they would hide corresponding classes in Applications.

Strictly speaking I believe that the "not package_name or " part
of the condition in the touched script is not needed because
LoadWhitelist() already skips empty lines and doesn't add "^$" to
the whitelist regex, but relying on this seems very fragile. If
there ever is a need to have classes in the bootclasspath's
unnamed package in future then we can always change this again.

Bug: 147480264
Test: Treehugger

Change-Id: Ic310dd0779dde133b3a5c3039ea5b70d31331a9b