[code-reflection] RFR: Build extern type element tree [v2]

[Paul Sandoz]

> The model that builds the model now explicitly builds the externalized type element tree rather than parsing it from the string form. Externalized type element instances are reused.

Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:

  Remove unused import

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Changes:
  - all: https://git.openjdk.org/babylon/pull/419/files
  - new: https://git.openjdk.org/babylon/pull/419/files/7648d19e..f9f9cb20

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=419&range=01
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=419&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/419.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/419/head:pull/419

PR: https://git.openjdk.org/babylon/pull/419