RFR: 6909: Upgrade to Eclipse 2020-09 [v3]

[Marcus Hirt]

> Adds the Eclipse 2020-09 platform, upgrades 2020-09 and 2020-06 to use the 2020-06 babel bundles, and sets 2020-09 to
> be the default.

Marcus Hirt has updated the pull request incrementally with one additional commit since the last revision:

  Switching to Tycho 2.0.0 for building, also fixing some annoying unused imports

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/111/files
  - new: https://git.openjdk.java.net/jmc/pull/111/files/260ae180..9c8daec6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=111&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=111&range=01-02

  Stats: 17 lines in 7 files changed: 1 ins; 13 del; 3 mod
  Patch: https://git.openjdk.java.net/jmc/pull/111.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/111/head:pull/111

PR: https://git.openjdk.java.net/jmc/pull/111