Perl package: detect opcode support in externals (#38618)

Spack-installed Perl always has opcode support, but external Perl
installations might not. This commit adds a +opcode variant and
updates the external detection logic to check for opcode support.

The postgresql package is updated to require perl+opcode (in
combination with the above, this helps detect when an external
Perl instance is sufficient for a Spack build of postgreqsql, or
if Spack needs to build its own Perl).

var/spack/repos/builtin/packages/perl/package.py

@@ -179,6 +179,7 @@ class Perl(Package):  # Perl doesn't use Autotools, it should subclass Package
     variant("shared", default=True, description="Build a shared libperl.so library")
     variant("threads", default=True, description="Build perl with threads support")
     variant("open", default=True, description="Support open.pm")
+    variant("opcode", default=True, description="Support Opcode.pm")
 
     resource(
         name="cpanm",
@@ -241,6 +242,10 @@ def determine_variants(cls, exes, version):
                 fail_on_error=False,
             )
             variants += "+open" if perl.returncode == 0 else "~open"
+            # this is just to detect incomplete installs
+            # normally perl installs Opcode.pm
+            perl("-e", "use Opcode", output=os.devnull, error=os.devnull, fail_on_error=False)
+            variants += "+opcode" if perl.returncode == 0 else "~opcode"
             return variants
 
     # On a lustre filesystem, patch may fail when files

var/spack/repos/builtin/packages/postgresql/package.py

@@ -57,7 +57,7 @@ class Postgresql(AutotoolsPackage):
     depends_on("libedit", when="lineedit=libedit")
     depends_on("openssl")
     depends_on("tcl", when="+tcl")
-    depends_on("perl", when="+perl")
+    depends_on("perl+opcode", when="+perl")
     depends_on("python", when="+python")
     depends_on("libxml2", when="+xml")