Support new perl module namespace in stable branches

Commit b3b4d8e68a moved our perl test modules to a better namespace
structure, but this has made life hard for people wishing to backpatch
improvements in the TAP tests. Here we alleviate much of that difficulty
by implementing the new module names on top of the old modules, mostly
by using a little perl typeglob aliasing magic, so that we don't have a
dual maintenance burden. This should work both for the case where a new
test is backpatched and the case where a fix to an existing test that
uses the new namespace is backpatched.

Reviewed by Michael Paquier

Per complaint from Andres Freund

Discussion: https://postgr.es/m/20220418141530.nfxtkohefvwnzncl@alap3.anarazel.de

Applied to branches 10 through 14

src/test/perl/PostgresNode.pm

@@ -2262,4 +2262,18 @@ sub corrupt_page_checksum
 
 =cut
 
+# support release 15+ perl module namespace
+
+package PostgreSQL::Test::Cluster; ## no critic (ProhibitMultiplePackages)
+
+sub new
+{
+	shift; # remove class param from args
+	return PostgresNode->get_new_node(@_);
+}
+
+no warnings 'once';
+
+*get_free_port = *PostgresNode::get_free_port;
+
 1;