RFR: 7576: Remove the need for a local jetty server for dependencies

[Austin Brooks]

Using newer Tycho features to achieve result:

Maven dependencies in a target (Tycho 2.2 ): https://github.com/eclipse/tycho/blob/master/RELEASE_NOTES.md#support-for-m2e-pde-maven-target-locations
Nested target to cut down on duplicate third party code (Tycho 2.6): https://github.com/eclipse/tycho/blob/master/RELEASE_NOTES.md#support-for-nested-targets

Note: Due to change in 2.7, you must delete a file when switching branches due to changes in the `.m2` folder: https://github.com/eclipse/tycho/blob/master/RELEASE_NOTES.md#caution-when-switching-between-tycho-versions

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Commit messages:
 - Remove references to third-party from scripts/
 - Fix clean target trying to access third-party directory
 - XXX: Remove needing to use a local jetty server for dependencies

Changes: https://git.openjdk.java.net/jmc/pull/387/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=387&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7576
  Stats: 481 lines in 12 files changed: 130 ins; 346 del; 5 mod
  Patch: https://git.openjdk.java.net/jmc/pull/387.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/387/head:pull/387

PR: https://git.openjdk.java.net/jmc/pull/387