fix(scss): import _input-group.scss once at most (#2239)

Prevents the CSS from appearing in the compiled output CSS more than once.

Reduces file size of generated bootstrap-vue.css

Author: tmorehouse

src/components/input-group/_input-group.scss

@@ -1,22 +1,29 @@
-/* workaround for https://github.com/bootstrap-vue/bootstrap-vue/issues/1560 */
-/* workaround for https://github.com/bootstrap-vue/bootstrap-vue/issues/2114 */
-/* source: bootstrap _input-group.scss */
-.input-group {
-  > .input-group-prepend > .btn-group,
-  > .input-group-append:not(:last-child) > .btn-group,
-  > .input-group-append:last-child > .btn-group:not(:last-child):not(.dropdown-toggle) {
-    > .btn {
-      border-top-right-radius: 0;
-      border-bottom-right-radius: 0;
+$bv-input-group-dropdown-patched: false !default;
+
+@if $bv-input-group-dropdown-patched == false {
+  // Only include this style patch once in final CSS
+  $bv-input-group-dropdown-patched: true;
+
+  /* workaround for https://github.com/bootstrap-vue/bootstrap-vue/issues/1560 */
+  /* workaround for https://github.com/bootstrap-vue/bootstrap-vue/issues/2114 */
+  /* based on: bootstrap/scss/_input-group.scss */
+  .input-group {
+    > .input-group-prepend > .btn-group,
+    > .input-group-append:not(:last-child) > .btn-group,
+    > .input-group-append:last-child > .btn-group:not(:last-child):not(.dropdown-toggle) {
+      > .btn {
+        border-top-right-radius: 0;
+        border-bottom-right-radius: 0;
+      }
     }
-  }
 
-  > .input-group-append > .btn-group,
-  > .input-group-prepend:not(:first-child) > .btn-group,
-  > .input-group-prepend:first-child > .btn-group:not(:first-child) {
-    > .btn {
-      border-top-left-radius: 0;
-      border-bottom-left-radius: 0;
+    > .input-group-append > .btn-group,
+    > .input-group-prepend:not(:first-child) > .btn-group,
+    > .input-group-prepend:first-child > .btn-group:not(:first-child) {
+      > .btn {
+        border-top-left-radius: 0;
+        border-bottom-left-radius: 0;
+      }
     }
   }
 }