Gradle: Replace legacy buildSrc mechanism with included build (#1524)

Motivation:
buildSrc is a special-case legacy mechanism.
Gradle recommends using an included build named build-logic instead:

https://docs.gradle.org/current/userguide/best_practices_structuring_builds.html#favor_composite_builds

Changes:
- Rename buildSrc/ to build-logic/
  - triggers reformatting
- Replace occurrences of "buildSrc" with "build-logic"
- Include the build-logic build in the main build (via
settings.gradle.kts)
- Apply convention plugins via plugin IDs instead of type-safe accessors
  - small tradeoff compared to buildSrc

Result:
- Faster and more isolated builds
- Build logic behaves like a normal build, making it easier to evolve
and reason about

---------

Co-authored-by: Daniel Chao <dan.chao@apple.com>

pkl-core/pkl-core.gradle.kts

@@ -17,10 +17,10 @@ import org.apache.tools.ant.filters.ReplaceTokens
 
 plugins {
   kotlin("jvm") // for `src/generator/kotlin`
-  pklAllProjects
-  pklJavaLibrary
-  pklPublishLibrary
-  pklNativeLifecycle
+  id("pklAllProjects")
+  id("pklJavaLibrary")
+  id("pklPublishLibrary")
+  id("pklNativeLifecycle")
   idea
 }