Splitting buildSrc sources into multiple directories

the first step towards multiple projects, multiple plugins, and multiple classpaths containing classes in buildSrc.

This will eventually mean that changes in buildSrc/impl won't need to invalidate the execution of tasks coming from externally applied plugins

Bug: 140265324

Test: Treehugger runs busytown/androidx.sh
Change-Id: I81af10d448c3d7a4055992e55b461b6ba986a258
diff --git a/buildSrc/plugins/README.md b/buildSrc/plugins/README.md
new file mode 100644
index 0000000..68b1cc4
--- /dev/null
+++ b/buildSrc/plugins/README.md
@@ -0,0 +1,3 @@
+This is the :buildSrc:plugins project
+
+It contains plugins to be applied by various other projects in this repository