buildSrc splitting imported sources into real subprojects

This reduces the time to run `./gradlew :help --clean` from:
  2m3s ( https://ge.androidx.dev/s/sndofud7jsaga/timeline )
to
  1m51s ( https://ge.androidx.dev/s/cqfbrmprl5qy2/timeline )

This also makes it easier to see that about 19s of the remaining time is caused by :buildSrc:imports:stableaidl-gradle-plugin:compileJava

Test: Treehugger runs busytown/*.sh

Test: cd room && ANDROID_HOME=../../../prebuilts/fullsdk-linux gw projects --no-daemon

Bug: 301121474

Change-Id: If144af509977c20a6e791643869eb188905dc178
diff --git a/buildSrc/plugins/README.md b/buildSrc/plugins/README.md
index 68b1cc4..363e1b4 100644
--- a/buildSrc/plugins/README.md
+++ b/buildSrc/plugins/README.md
@@ -1,3 +1,5 @@
 This is the :buildSrc:plugins project
 
 It contains plugins to be applied by various other projects in this repository
+
+The plugins in this project do not get published to remote repositories