[MASSEMBLY-832] Consider dependency optional attribute when gather de…

[zabetak]

https://issues.apache.org/jira/browse/MASSEMBLY-832

Maven allows to declare dependencies as optional and this is important for various reasons (e.g., avoiding licensing problems).

This change provides a convenient way to include/exclude optional dependencies in a dependencySet.

By default optional dependencies are excluded from the dependencySet. An optional dependency shouldn't be packaged, distributed, or transitively inherited unless specified explicitly so it makes sense to exclude them by default.

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