RFR: 8297437: javadoc cannot link to old docs (with old style anchors)

[Roman Marchenko]

As it discussed on the mail list, checking presence of "element-list" or "package-list" is good enough approach to determine which type of anchors is used in external docs.

This fix extends `Extern.Item` with `useOldFormId` boolean flag to indicate a type of anchor. The boolean flag is set accordingly when reading a list of elements from either "element-list" or "package-list". 

Name transformation is performed in `Extern.getExternalLink()` by `getOldFormHtmlName()` call when creating `DocLink `instance.

Type of anchors used by platform docs are hard-coded in `isOldFormPlatformDocs()`, since it's not supposed to rebuild platform docs on docs.oracle.com.

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Commit messages:
 - Fixed creating links to old-form anchors

Changes: https://git.openjdk.org/jdk/pull/12066/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12066&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297437
  Stats: 137 lines in 6 files changed: 104 ins; 0 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/12066.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12066/head:pull/12066

PR: https://git.openjdk.org/jdk/pull/12066