RFR: 8302819: Remove JAR Index [v6]

[Eirik Bjorsnos]

> This PR removes the JAR index feature from the runtime:
> 
> - `URLClassPath` is  updated to remove the `enableJarIndex` system property and any code which would be called when this property was `true`
> - The `JarIndex` implementation class is moved into `jdk.jartool` module.
> - The `InvalidJarIndexError` exception class is removed because it falls out of use
> - The test `test/jdk/sun/misc/JarIndex/metaInfFileNames/Basic.java` is removed because it depends on the JarIndex feature being present
> - The test `test/jdk/sun/misc/JarIndex/JarIndexMergeForClassLoaderTest.java` is removed because it depends on the JarIndex feature being present
> - The test `test/jdk/sun/misc/JarIndex/JarIndexMergeTest.java` is removed because it end up being the only caller of the  JarIndex.merge feature
> - All `JarIndex` methods/constructors which are not used by the `jar -i` implementation are removed. 
> - `JarIndex` is given package-private access.

Eirik Bjorsnos has updated the pull request incrementally with one additional commit since the last revision:

  Add a note in the --help for --generate-index that this option is deprecated and may be removed in a future release.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13158/files
  - new: https://git.openjdk.org/jdk/pull/13158/files/79b19e94..d04df25e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13158&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13158&range=04-05

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13158.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13158/head:pull/13158

PR: https://git.openjdk.org/jdk/pull/13158