RFR: 8352858: Make java.net.JarURLConnection fields final [v2]

[Eirik Bjørsnøs]

> Please help review this cleanup PR which makes the `java.net.JarURLConnection` fields `jarFileURL` and `entryName` final.
> 
> The current `parseSpec` method is somewhat crufty, with some code quality  issues which this PR aims to improve:
> 
> * The method-level comment seems stale and misplaced
> * The local variable  `separator` is confusingly incremented during parsing (using pre AND post increment operators) 
> * Unused local variables introduced for the sole purpose of attaching `@SuppressWarnings` annotations
> * The `jarFileURL` and `entryName` fields are both assigned more than once
> * Block comments are used where line comments would suffice
> 
> The PR addresses the above issues by inlining `parseSpec` into the constructor, then extracting static helper methods for parsing the file URL and the entry name. This allows the fields to be made final.
> 
> Since this is purely a refactoring PR, no tests are updated and the `noreg-cleanup` label is added in JBS.
> 
> Reviewing individual commits in this PR may aid verification of separate refactorings.

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

  Rename 'separator' to 'separatorIndex', 'nameIdx' to 'nameIndex'

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24218/files
  - new: https://git.openjdk.org/jdk/pull/24218/files/1f9fc452..ddcb18d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24218&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24218&range=00-01

  Stats: 12 lines in 1 file changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/24218.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24218/head:pull/24218

PR: https://git.openjdk.org/jdk/pull/24218