Syntax for external imports

[yannham]

As part of #1585, propose and bikeshed a new syntax to import files/modules (although the latter doesn't have a proper definition yet) from an external package manager. This could be something such as import foo from nix, import npm:foo, etc.

Keep in mind that we might also reserve some syntax to explicitly specify the format of the imported file (such as json or yaml), independently from the source. We might want a syntax which can accommodate both, such as e.g. import foo from npm as yaml.