Fix relative module resolution in fatjar URLs

[GuntherRademacher]

The changes that were done in the context of moving on to Java 21 brought up a regression in the code that supports fatjar loading (originally added in #2154). Commit 2d905d8 replaced the deprecated new URL(new URL(pth), path) with URI.resolve:

return get(JARPREF + new URI(pth.substring(JARPREF.length())).resolve(path));

The problem: for a plain jar URL like jar:file:/a/b.jar!/…, stripping jar: leaves file:/a/b.jar!/…, which is a hierarchical URI, so resolve merges paths correctly. But a fatjar URL nests a custom scheme - jar:fatjar:WEB-INF/lib/rr.jar!/…. Stripping jar: leaves fatjar:WEB-INF/…, whose scheme-specific part doesn't start with /, making it an opaque URI. URI.resolve against an opaque base discards the base entirely and just returns the relative reference.

The old new URL(...) path never had this issue because the jar: URL handler splits at !/ and resolves only the entry.

Consequently the fix is to check for the presence of !/, keep the jar-location prefix (jar:…!/) intact and resolve the relative reference only against the entry path, which is always hierarchical.