fix(MakeMaker): use :: rules so Alien::Build postambles work

[fglock]

Summary

Fixes two distinct bugs exposed by jcpan -t Net::SSH::Perl.

1. Mixed : / :: rules in generated Makefile

make for Alien::m4 and Alien::GMP (deps of Math::GMP, required
by Net::SSH::Perl) died with:

Makefile:60: *** target file `realclean' has both : and :: entries.  Stop.

Root cause: our PerlOnJava MakeMaker template emitted clean:,
realclean:, distclean:, all:, pm_to_blib:, pure_all:, test:,
install: with single-colon rules. Postambles from Alien::Build
(MY::postamble) and File::ShareDir::Install extend the same targets
with double-colon rules — which GNU make rejects when mixed. Real
ExtUtils::MakeMaker uses :: on these targets for exactly this
reason. Switched our template to match.

2. Closure-capture bug with eval STRING in a sibling sub

Minimal reproducer — both system perl 5.42 and jperl should print
keys=NAME,X:

use strict; use warnings;
foo();                                  # forward call, does eval STRING
my %WM = ( NAME => 'Foo', X => 'Y' );   # declared AFTER the call
do_it();
sub do_it { print "keys=", join(",", sort keys %WM), "\n"; }
sub foo   { eval("1;"); }

Before this PR jperl printed keys= (empty). This caused
Net::SSH::Perl's modulino Makefile.PL to die with NAME is required
whenever Digest::BubbleBabble was already installed — the
have_module() path then runs eval "use ..." BEFORE the file-scope
my %WriteMakefile = (...) assignment.

Mechanism. Named/package subs capture outer my lexicals via a
"BEGIN-package alias":

• SubroutineParser.handleNamedSub registers the shared
  RuntimeHash/Array/Scalar under PerlOnJava::_BEGIN_<id>::<name>
  at compile time, and passes it as a constructor arg to the named sub's
  generated class.
• At runtime, handleMyOperator calls PersistentVariable.retrieveBeginHash
  which takes ownership of the same object out of the alias map — so
  the my variable and the named sub's capture point to the same
  RuntimeHash.

RuntimeCode.evalStringHelper also populates that same alias map at
entry (so BEGIN blocks inside the eval can see outer lexicals) and
unconditionally removes the entries in its finally block. When a
sibling sub ran eval STRING before the outer my had executed, the
finally block removed SubroutineParser's compile-time alias. The later
my %WM = (...) then found nothing in the map, created a fresh
RuntimeHash, and the named sub's closure was left pointing at an
empty, orphaned object.

Fix. Only record an alias for cleanup in evalAliasKeys if this
invocation was the one that actually installed it (putIfAbsent
returns null). If a compile-time alias was already there, leave it
alone; its owner — the outer my at runtime, or an enclosing eval —
will clean it up. Both the JVM path and the BytecodeInterpreter path in
evalStringHelper had the same bug and get the same fix.

Regression test added at src/test/resources/unit/closure_eval_string.t
covering hash, array and scalar closures with the exact shape from
Net::SSH::Perl's Makefile.PL. Behaviour verified identical on
system perl 5.42.0 and jperl.

Additional, not in this PR

Also visible in the same jcpan -t Net::SSH::Perl run:

• ExtUtils::CBuilder / Alien::Build invokes javac as a C compiler
  because Config{cc} = 'javac'. Produces nonsense probe commands like
  javac -Iperlonjava/.../CORE -c -DSILENT_NO_TAINT_SUPPORT ... mytest.c.
  We should either set cc to something obviously absent so probes
  fail cleanly, or short-circuit CBuilder probes.
• XS-only hard deps reported "missing" but install continues.
  Net::SSH::Perl's hard deps (CryptX, Crypt::Curve25519,
  Crypt::IDEA, String::CRC32) have no pure-Perl fallback, so
  installing the .pm files "anyway" just produces cryptic
  Can't load loadable object for module CryptX at test time. Worth
  failing Makefile.PL when hard runtime deps lack both an XS Java
  bridge and a pure-Perl fallback.

Test plan

• make passes (full unit-test suite including the new
  closure_eval_string.t).
• ./jcpan -t Alien::m4 progresses past the Makefile parse error.
• Net::SSH::Perl's Makefile.PL now completes with
  "Configured! 75 files will be installed".
• Minimal closure repros produce keys=NAME,X on both system
  perl 5.42 and jperl.
• Refreshed dev/cpan-reports/ with the latest random-tester run.