Build Gradle plugin from source and fix configuration-cache logging

[rossbacher]

The repository's root buildscript depends on its own published deeplinkdispatch-gradle-plugin:7.3.0-SNAPSHOT. When that snapshot is unavailable, even the commands needed to build, test, or publish the plugin fail during configuration.

Bootstrap the existing plugin and base-library sources through buildSrc, and remove the published-plugin buildscript dependency. The samples now use the plugin from the current checkout without a preliminary publication. The existing plugin project retains its test and publishing tasks, while dependency versions and JVM settings come from dependencies.gradle. Update the sample comments and contributor documentation to describe this workflow.

Also fix the missing-manifest diagnostic in RelocateDeepLinkManifestTask: use the task's path instead of accessing project.name during execution. A regression test verifies both configuration-cache storage and reuse with the task action forced to execute each time.

Context: https://airbnb.slack.com/archives/DA5UX38NP/p1790030644332959

Validation

• Normal repository build: all 13 Gradle plugin tests passed, and :sample-ksp-library:assembleDebug succeeded using the local plugin (--no-build-cache).
• The configuration-cache regression test fails against the original logging code with the reported Task.project error.
• The existing -PenforceSnapshotVersion=true publishAllPublicationsToMavenCentral command resolves successfully with --dry-run --offline; no artifacts were uploaded.

Review

@airbnb/android-platform