Support registration of non-public BeanDefinitionReader via @⁠ImportResource

Prior to this commit, a BeanDefinitionReader registered via
@⁠ImportResource was required to be public and have a public
constructor that accepts a single BeanDefinitionRegistry. However, the
public visibility requirements are not necessary, and the requirements
for the constructor's formal parameter list is not documented.

To address those issues, this commit removes the public visibility
restrictions and documents that a BeanDefinitionReader registered via
@⁠ImportResource must declare a constructor that accepts a single
BeanDefinitionRegistry.

In addition, this commit includes the cause of the instantiation
failure in case the registered BeanDefinitionReader cannot be
instantiated.

Closes gh-34928

spring-context/src/test/java/org/springframework/context/annotation/configuration/ImportResourceTests.java

@@ -23,6 +23,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.aop.support.AopUtils;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Value;
+import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.testfixture.beans.TestBean;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
@@ -109,6 +110,13 @@ class ImportResourceTests {
 		}
 	}
 
+	@Test
+	void importResourceWithPrivateReader() {
+		try (AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(ImportWithPrivateReaderConfig.class)) {
+			assertThat(ctx.containsBean("propertiesDeclaredBean")).isTrue();
+		}
+	}
+
 
 	@Configuration
 	@ImportResource("classpath:org/springframework/context/annotation/configuration/ImportXmlConfig-context.xml")
@@ -173,4 +181,20 @@ class ImportResourceTests {
 	static class ImportNonXmlResourceConfig {
 	}
 
+	@SuppressWarnings("deprecation")
+	@Configuration
+	@ImportResource(locations = "org/springframework/context/annotation/configuration/ImportNonXmlResourceConfig.properties",
+			reader = PrivatePropertiesBeanDefinitionReader.class)
+	static class ImportWithPrivateReaderConfig {
+	}
+
+	@SuppressWarnings("deprecation")
+	private static class PrivatePropertiesBeanDefinitionReader
+			extends org.springframework.beans.factory.support.PropertiesBeanDefinitionReader {
+
+		PrivatePropertiesBeanDefinitionReader(BeanDefinitionRegistry registry) {
+			super(registry);
+		}
+	}
+
 }