Require the package name to prefix all imports

[jfecher]

Part of implementing the "Packages" design: https://antelang.org/docs/language/#packages

Currently import paths to the stdlib (https://github.com/jfecher/ante/tree/master/stdlib) can be imported via import Vec where we want the package name to be prefixed in the path: import Std.Vec to allow for future additions to the stdlib without conflicting with any names in existing programs. This should also apply to the user's current project. E.g. in MyProject/{T.an, U.an} the imports import T and import U are currently allowed, where these will need to be import MyProject.T in the future instead.