Cherry-pick PR microsoft#50572 into release-4.8

Component commits:
5e96e58 Lets make the importServicePlugin internal for now?

Author: sheetalkamat

src/server/types.ts

@@ -20,6 +20,7 @@ declare namespace ts.server {
         gc?(): void;
         trace?(s: string): void;
         require?(initialPath: string, moduleName: string): ModuleImportResult;
+        /*@internal*/
         importServicePlugin?(root: string, moduleName: string): Promise<ModuleImportResult>;
     }
 }

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -7033,7 +7033,6 @@ declare namespace ts.server {
         gc?(): void;
         trace?(s: string): void;
         require?(initialPath: string, moduleName: string): ModuleImportResult;
-        importServicePlugin?(root: string, moduleName: string): Promise<ModuleImportResult>;
     }
 }
 declare namespace ts.server {