Display the correct library imports for all packages on pub.dartlang.org

[nex3]

Currently, the package view pub.dartlang.org always displays the example import as 'import "package:packagename/packagename.dart"'. Some packages don't have a library with the same name as the package, though, and for those packages this import example will fail. We should display one or more valid imports for these packages instead, or at least an import string that has an obvious placeholder.

[nex3]

Added Started label.

[nex3]

56d2782 makes this work for new package versions, but it still needs to be applied to older ones.

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Marked this as being blocked by #5792.

[nex3]

Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#187.