Use modifiable set for @ServletComponentScan with no packages

Previously, when a project contained multiple `@ServletComponentScan`
annotated classes in classpath, and at least one annotation don't
explicitly specify `basePackages` and `basePackageClass` attribute,
the application could fail to start with an
UnsupportedOperationException. The failure occurred due to the
creating of an unmodifiable set when no base packages are configured
and a subsequent attempt to add base packages to that sit.

This commit fixes the issue by removing the use of an unmodifiable set
when `@ServletComponentScan` with no base packages in processed before
any other `@ServletComponentScan` annotations.

See gh-12715

spring-boot/src/main/java/org/springframework/boot/web/servlet/ServletComponentScanRegistrar.java

@@ -17,7 +17,6 @@
 package org.springframework.boot.web.servlet;
 
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.Set;
 
@@ -84,8 +83,7 @@ class ServletComponentScanRegistrar implements ImportBeanDefinitionRegistrar {
 			packagesToScan.add(ClassUtils.getPackageName(basePackageClass));
 		}
 		if (packagesToScan.isEmpty()) {
-			return Collections
-					.singleton(ClassUtils.getPackageName(metadata.getClassName()));
+			packagesToScan.add(ClassUtils.getPackageName(metadata.getClassName()));
 		}
 		return packagesToScan;
 	}