RFR: 8352858: Make java.net.JarURLConnection fields final

[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.

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Commit messages:
 - Inline parseSpec into the constructor, make fields final
 - Extract parseJarFileURL method
 - Extract parseEntryName method
 - Make separator local variable effectively final, assign entryName once

Changes: https://git.openjdk.org/jdk/pull/24218/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352858
  Stats: 57 lines in 1 file changed: 31 ins; 9 del; 17 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