[MNG-6776] Inconsistent list of parameters for MojoDescriptor (#584)

[michael-o]

It may lead to confusion as parameters (list) and
parametersMap (map) may "fall apart" easily.

Also, parametersMap did not honor parameter
ordering while parameters list makes it look
that order is important.
Simply, rebuild the map always and retain
ordering as well.

Added UT and also removed some cruft provided
by java8.

Co-authored-by: Marc Wrobel marc.wrobel@gmail.com
Co-authored-by: Slawomir Jaranowski s.jaranowski@gmail.com

(cherry picked from commit cc51006) (cherry picked from commit 5ce5591) (cherry picked from commit 47e2d68) ---

https://issues.apache.org/jira/browse/MNG-7309

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[michael-o]

@slachiewicz As requested...