Move tool versions to Version Catalog

- fix: make version catalog accessible from `buildSrc` plugins
- chore: declare `googleFormatVersion` in version catalog
- chore: declare `ktfmt` in version catalog

Relates-To: apple/pkl#204
Signed-off-by: Sam Gammon <sam@elide.ventures>

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

@@ -1,3 +1,5 @@
+import org.gradle.accessors.dm.LibrariesForLibs
+
 plugins {
   id("pklJavaLibrary")
 
@@ -6,6 +8,9 @@ plugins {
 
 val buildInfo = project.extensions.getByType<BuildInfo>()
 
+// Version Catalog library symbols.
+val libs = the<LibrariesForLibs>()
+
 dependencies {
   // At least some of our kotlin APIs contain Kotlin stdlib types
   // that aren't compiled away by kotlinc (e.g., `kotlin.Function`).
@@ -21,7 +26,7 @@ tasks.compileKotlin {
 
 spotless {
   kotlin {
-    ktfmt("0.44").googleStyle()
+    ktfmt(libs.versions.ktfmt.get()).googleStyle()
     targetExclude("**/generated/**", "**/build/**")
     licenseHeaderFile(rootProject.file("buildSrc/src/main/resources/license-header.star-block.txt"))
   }