[MSHADE-272] Option to preserve source timestamps

**[Philip Aston](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=philipa@mail.com)** opened **[MSHADE-272](https://issues.apache.org/jira/browse/MSHADE-272?redirect=false)** and commented

Please add an option to preserve the source timestamps of files.

Not doing so causes a significant issue when packaging Clojure projects, since Clojure uses the timestamps to determine whether to dynamically recompile code. See:

- http://mail-archives.apache.org/mod_mbox/maven-users/201310.mbox/%3CCAAuB-epb4cdpUYmGEPu7tHsB2bqCUjmgmnm1B4h1Ucmyr+UhVw@mail.gmail.com%3E

- https://groups.google.com/d/msg/clojure/12BOjg1KsWA/8DIobmb2EwAJ

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**Affects:** 3.1.0

The issue names no source files, tests, or entry points. Begin by tracing how the Maven Shade Plugin packages files and find existing option or configuration tests; completion should include an opt-in way to preserve source timestamps, with the behavior verified by tests.

Written by the indexing model from the issue text.