Change the way imports are done to support metadata more cleanly

[JonathanAldrich]

Right now ImportDeclaration.genBinding extends the context with a shortcut replacing the importName with the long, internalName for the module. But we want the importName to not be a synonym, because we only want its type to have metadata if we used "import metadata" at the source level. So should really replace this with a binding that doesn't replace the importName with the internalName, and a corresponding "let" declaration later.

This is a bug because it means "import metadata" is effectively ignored (metadata is always imported).