RFR: 8323746: Add PathElement hashCode and equals [v3]

[Per Minborg]

> This PR proposes to implement `hashCode()` and `equals()` methods for implementations of `PathElement`.
> 
> In doing so, the previous `PathElementImpl` was removed and replaced in favor of distinct `record` implementations, each reflecting its own path element selection type. This also allowed the `PathKind` to be removed as this piece of information is now carried in the sealed type hierarchy. 
> 
> It is worth noting, the implementations resides in the `jdk.internal` package and consequently, they are not exposed to clients. So, we could use pattern matching (for example) internally but not in client code.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Rework hashCode implementations and add a test for hash codes

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17651/files
  - new: https://git.openjdk.org/jdk/pull/17651/files/5685e31a..d6da6221

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17651&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17651&range=01-02

  Stats: 15 lines in 2 files changed: 9 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17651.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17651/head:pull/17651

PR: https://git.openjdk.org/jdk/pull/17651