[foreign-jextract] RFR: 8253430: Improve the LibraryLookup API (jextract version)

[Maurizio Cimadamore]

This patch tweaks jextract's RuntimeHelper to use the updated LibraryLookup signatures.

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Commit messages:
 - Merge branch 'foreign-jextract' into lookup-optional+jextract
 - Tweak signature of LibraryLookup::ofPath to take a Path argument
 - Merge branch 'foreign-jextract' into lookup-optional+jextract
 - Tweak LibraryLookup::lookup to return optional

Changes: https://git.openjdk.java.net/panama-foreign/pull/349/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=349&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253430
  Stats: 31 lines in 3 files changed: 4 ins; 18 del; 9 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/349.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/349/head:pull/349

PR: https://git.openjdk.java.net/panama-foreign/pull/349