'Create Library Packages' page - document that library directives should be unique

[natebosch]

Page URL: https://www.dartlang.org/guides/libraries/create-library-packages
Page source: https://github.com/dart-lang/site-www/tree/master/src/_guides/libraries/create-library-packages.md

Until we get dart-lang/dartdoc#1082 resolved there is still a need for many libraries to have a library directive.

In Dart you can't import two libraries with the same name - so we should call out that if you're using a library directive in a package you distribute the library directive should always be unique - like package_name.something rather than something.

[parlough]

I know this is four years late...But do you think we need to mention this now that no-name library declarations will be allowed in 2.19? Would it be sufficient to just expand on our documentation on how to use a no-name library declaration and add documentation to that (tracked in #4314)?

[parlough]

I'm going to close this as the library name clash rule was removed from the spec in dart-lang/language#1083 and the analyzer removed the warning in dart-lang/sdk@ef147c0.