fix(material/core): Fix legacy prebuilt themes export (#26624)

(cherry picked from commit 6641004)

Author: lukasmatta

src/material/package-base.json

@@ -38,16 +38,16 @@
       "style": "./prebuilt-themes/*.css"
     },
     "./legacy-prebuilt-themes/indigo-pink.css": {
-      "style": "./legacy-prebuilt-themes/indigo-pink.css"
+      "style": "./legacy-prebuilt-themes/legacy-indigo-pink.css"
     },
     "./legacy-prebuilt-themes/deeppurple-amber.css": {
-      "style": "./legacy-prebuilt-themes/deeppurple-amber.css"
+      "style": "./legacy-prebuilt-themes/legacy-deeppurple-amber.css"
     },
     "./legacy-prebuilt-themes/pink-bluegrey.css": {
-      "style": "./legacy-prebuilt-themes/pink-bluegrey.css"
+      "style": "./legacy-prebuilt-themes/legacy-pink-bluegrey.css"
     },
     "./legacy-prebuilt-themes/purple-green.css": {
-      "style": "./legacy-prebuilt-themes/purple-green.css"
+      "style": "./legacy-prebuilt-themes/legacy-purple-green.css"
     },
     "./legacy-prebuilt-themes/*": {
       "style": "./legacy-prebuilt-themes/*.css"