backend-app-api: fixes, renames, cleanup

Co-authored-by: Johan Haals <johan.haals@gmail.com>
Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/backend-plugin-api/README.md

@@ -1,5 +1,7 @@
 # @backstage/backend-plugin-api
 
+**This package is HIGHLY EXPERIMENTAL, do not use this for production**
+
 This package provides the core API used by Backstage backend plugins.
 
 ## Installation