Change loading of external readers (#1394)

This introduces breaking changes for external readers are loaded:

1. In PklProject, relative paths are resolved relative to the enclosing
PklProject file (make behavior consistent with how other settings work)
2. Make CLI flags blow away any settings set on a PklProject
3. Introduce a new `workingDir` property, which defaults to the
PklProject dir

The overall goal is to make this behavior consistent with how other
settings work.
For example, relative paths for other evaluator settings are already
relative to the project directory.
Additionally, in every other case, CLI flags will overwrite any setting
set within PklProject.

pkl-core/pkl-core.gradle.kts

@@ -27,7 +27,17 @@ plugins {
   idea
 }
 
-val generatorSourceSet = sourceSets.register("generator")
+val generatorSourceSet: NamedDomainObjectProvider<SourceSet> = sourceSets.register("generator")
+
+val externalReaderFixtureSourceSet: NamedDomainObjectProvider<SourceSet> =
+  sourceSets.register("externalReaderFixture") {
+    compileClasspath += sourceSets.test.get().output + sourceSets.test.get().compileClasspath
+    runtimeClasspath += sourceSets.test.get().output + sourceSets.test.get().runtimeClasspath
+  }
+
+val externalReaderFixtureImplementation: Configuration by configurations.getting {
+  extendsFrom(configurations.testImplementation.get())
+}
 
 idea {
   module {
@@ -126,8 +136,45 @@ tasks.withType<JavaCompile>().configureEach {
 
 tasks.compileKotlin { enabled = false }
 
+val externalReaderFixture by tasks.registering {
+  group = "build"
+  dependsOn(tasks.named("compileExternalReaderFixtureJava"))
+  inputs.files(externalReaderFixtureSourceSet.map { it.output })
+  val fileName = if (buildInfo.os.isWindows) "externalreader.bat" else "externalreader"
+  val outputFile = layout.buildDirectory.file("fixtures/$fileName")
+  outputs.file(outputFile)
+  doLast {
+    val classpath = externalReaderFixtureSourceSet.get().runtimeClasspath.asPath
+    val scriptContent =
+      if (buildInfo.os.isWindows) {
+        """
+          @echo off
+          java -cp $classpath org.pkl.core.externalreaderfixture.Main
+        """
+          .trimIndent()
+      } else {
+        """
+          #!/usr/bin/env bash
+
+          java -cp $classpath org.pkl.core.externalreaderfixture.Main
+        """
+          .trimIndent()
+      }
+
+    outputFile.get().asFile.writeText(scriptContent)
+    outputFile.get().asFile.setExecutable(true)
+    println("Created external reader ${outputFile.get().asFile.absolutePath}")
+  }
+}
+
 tasks.test {
   configureTest()
+  dependsOn(externalReaderFixture)
+  environment(
+    "PATH",
+    listOf(System.getenv("PATH"), layout.buildDirectory.dir("fixtures/").get())
+      .joinToString(File.pathSeparator),
+  )
   useJUnitPlatform {
     excludeEngines("MacAmd64LanguageSnippetTestsEngine")
     excludeEngines("MacAarch64LanguageSnippetTestsEngine")
@@ -139,6 +186,11 @@ tasks.test {
 
   // testing very large lists requires more memory than the default 512m!
   maxHeapSize = "1g"
+
+  systemProperty(
+    "org.pkl.core.testExternalReaderPath",
+    externalReaderFixture.map { it.outputs.files.singleFile.absolutePath },
+  )
 }
 
 val generateBaseModuleMembers by