Update to Gradle 8.6 (#245)

- Fix and clean up the pkl-commons-test build script.
- Change tests to read test packages/certs directly from
  the file system instead of packaging and reading them
  from the class path.
- Update expected checksums of some test packages.
- Fix a conflict between Pkl's and Gradle's
  Kotlin libraries in the pkl-gradle project.
- Fix build deprecation warnings.
- Ensure Gradle distribution integrity with `distributionSha256Sum`.
- Manually verify integrity of Gradle wrapper added by this commit.

buildSrc/src/main/kotlin/pklFatJar.gradle.kts

@@ -105,7 +105,7 @@ tasks.check {
 }
 
 val validateFatJar by tasks.registering {
-  val outputFile = file("$buildDir/validateFatJar/result.txt")
+  val outputFile = file("build/validateFatJar/result.txt")
   inputs.files(tasks.shadowJar)
   inputs.property("nonRelocations", nonRelocations)
   outputs.file(outputFile)
@@ -138,7 +138,7 @@ tasks.check {
 
 val resolveSourcesJars by tasks.registering(ResolveSourcesJars::class) {
   configuration.set(configurations.runtimeClasspath)
-  outputDir.set(project.file("$buildDir/resolveSourcesJars"))
+  outputDir.set(project.file("build/resolveSourcesJars"))
 }
 
 val fatSourcesJar by tasks.registering(MergeSourcesJars::class) {

buildSrc/src/main/kotlin/pklHtmlValidator.gradle.kts

@@ -33,7 +33,7 @@ dependencies {
 }
 
 val validateHtml by tasks.registering(JavaExec::class) {
-  val resultFile = file("$buildDir/validateHtml/result.txt")
+  val resultFile = file("build/validateHtml/result.txt")
   inputs.files(htmlValidator.sources)
   outputs.file(resultFile)
 

buildSrc/src/main/kotlin/pklJavaLibrary.gradle.kts

@@ -55,7 +55,7 @@ val workAroundKotlinGradlePluginBug by tasks.registering {
     // A problem was found with the configuration of task ':pkl-executor:compileJava' (type 'JavaCompile').
     // > Directory '[...]/pkl/pkl-executor/build/classes/kotlin/main'
     // specified for property 'compileKotlinOutputClasses' does not exist.
-    file("$buildDir/classes/kotlin/main").mkdirs()
+    file("build/classes/kotlin/main").mkdirs()
   }
 }
 

buildSrc/src/main/kotlin/pklPublishLibrary.gradle.kts

@@ -54,7 +54,7 @@ val validatePom by tasks.registering {
     return@registering
   }
   val generatePomFileForLibraryPublication by tasks.existing(GenerateMavenPom::class)
-  val outputFile = file("$buildDir/validatePom") // dummy output to satisfy up-to-date check
+  val outputFile = file("build/validatePom") // dummy output to satisfy up-to-date check
 
   dependsOn(generatePomFileForLibraryPublication)
   inputs.file(generatePomFileForLibraryPublication.get().destination)