The behaviour I'd expect to see:

"module:qual.name" -> "imports module, accesses module.qual.name"
"module.qual.name" -> "no implicit import, accesses module.qual.name"
"package.submodule:qual.name" -> "imports package.submodule, accesses package.submodule.qual.name"
"package.submodule.qual.name" -> "no implicit import, accesses package.submodule.qual.name"

So if you have ":" in the capsule path, you're requesting that the interpreter execute an import with the path up to that point before attempting to resolve the full name reference.

By contrast, if you omit the ":", you're telling the interpreter that you'll take care of ensuring that any required imports have taken place before attempting to resolve the capsule reference.