Support core User::pwent and User::grent modules #1624

Summary

PerlOnJava cannot load the core modules User::pwent and User::grent, although both are part of standard Perl. This prevents CPAN distributions that use them from loading. The issue was exposed by Filesys-Virtual-Plain 0.10.

Reproduction

use User::pwent;
use User::grent;
print "ok\n";

Expected output: ok.

Actual output on both the JVM and interpreter backends: Can't locate User/pwent.pm in @INC.

The upstream Filesys-Virtual-Plain test t/00_use.t passes under system Perl 5.42.2 (1/1 tests). PerlOnJava's CPAN run fails before executing that test because User::pwent is unavailable; the direct load check also fails on both backends.

Environment and impact

Observed on macOS arm64 with Java 24.0.2, PerlOnJava commit a0b6a6256 (reported as Perl 5.44.1). CPAN tester run: 20261002-132816-78055; distribution: Filesys-Virtual-Plain 0.10.

MetaCPAN targeted reverse-dependency lookup found 8 unique released runtime dependants of User::pwent (2 recent): Sys-Export, App-Sqitch, Test-PostgreSQL, App-TimeTracker, DhMakePerl, App-wmiirc, vuser, and Filesys-Virtual-Plain. User::grent has 1 unique runtime dependant (0 recent): Filesys-Virtual-Plain.

Acceptance criteria

• use User::pwent and use User::grent load under both PerlOnJava backends.
• Add project regression coverage for the module loading behavior.