Disable module file generation by default (#37258)

* Disable module generation by default (#35564)

a) It's used by site administrators, so it's niche
b) If it's used by site administrators, they likely need to modify the config anyhow, so the default config only serves as an example to get started
c) it's too arbitrary to enable tcl, but disable lmod

* Remove leftover from old module file schema

* Warn if module file config is detected and generation is disabled

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Co-authored-by: Harmen Stoppels <harmenstoppels@gmail.com>

etc/spack/defaults/modules.yaml

@@ -40,9 +40,8 @@ modules:
     roots:
      tcl:    $spack/share/spack/modules
      lmod:   $spack/share/spack/lmod
-    # What type of modules to use
-    enable:
-      - tcl
+    # What type of modules to use ("tcl" and/or "lmod")
+    enable: []
 
     tcl:
       all: