Generating a library with no explicit prefix uses a default `lib` prefix

[filipesilva]

It should not default to any prefix.

This is related to angular/angular-cli#10405, and is blocking Angular CLI e2e tests.

[filipesilva]

Also related to #720

[filipesilva]

Talked with the team, and libraries should use the lib prefix if none is provided after all.

[filipesilva]

Fixed via angular/angular-cli#10405