Fix checkstyle import control

Use fully expanded referenced in an attempt to make the CI happy.

Fixes gh-8532

spring-boot-parent/pom.xml

@@ -18,7 +18,7 @@
 		<url>http://www.spring.io</url>
 	</organization>
 	<properties>
-		<main.basedir>..</main.basedir>
+		<main.basedir>${basedir}/..</main.basedir>
 		<disable.checks>false</disable.checks>
 		<java.version>1.8</java.version>
 		<aether.version>1.0.2.v20150114</aether.version>
@@ -502,7 +502,7 @@
 							<configLocation>src/checkstyle/checkstyle.xml</configLocation>
 							<suppressionsLocation>src/checkstyle/checkstyle-suppressions.xml</suppressionsLocation>
 							<headerLocation>src/checkstyle/checkstyle-header.txt</headerLocation>
-							<propertyExpansion>checkstyle.build.directory=${project.build.directory}</propertyExpansion>
+							<propertyExpansion>main.basedir=${main.basedir}</propertyExpansion>
 							<encoding>UTF-8</encoding>
 							<consoleOutput>true</consoleOutput>
 							<failsOnError>true</failsOnError>

spring-boot-parent/src/checkstyle/checkstyle.xml

@@ -78,7 +78,7 @@
 			<property name="processJavadoc" value="true" />
 		</module>
 		<module name="com.puppycrawl.tools.checkstyle.checks.imports.ImportControlCheck">
-			<property name="file" value="spring-boot-parent/src/checkstyle/import-control.xml" />
+			<property name="file" value="file://${main.basedir}/spring-boot-parent/src/checkstyle/import-control.xml" />
 			<property name="path" value="^.*[\\/]src[\\/]main[\\/].*$" />
 		</module>
 		<module name="com.puppycrawl.tools.checkstyle.checks.imports.ImportOrderCheck">