remove various little backend-common references

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

docs/backend-system/core-services/database.md

@@ -20,7 +20,7 @@ import {
   coreServices,
   createBackendPlugin,
 } from '@backstage/backend-plugin-api';
-import { resolvePackagePath } from '@backstage/backend-common';
+import { resolvePackagePath } from '@backstage/backend-plugin-api';
 
 createBackendPlugin({
   pluginId: 'example',

docs/contribute/project-structure.md

@@ -82,9 +82,11 @@ are separated out into their own folder, see further down.
   package. The `backend` uses plugins to construct a working backend that the
   frontend (`app`) can use.
 
-- [`backend-common/`](https://github.com/backstage/backstage/tree/master/packages/backend-common) -
-  There are no "core" packages in the backend. Instead we have `backend-common`
-  which contains helper middleware and other utils.
+- [`backend-app-api/`](https://github.com/backstage/backstage/tree/master/packages/backend-app-api) -
+  This package contains the central wiring for how to make Backstage backends.
+
+- [`backend-plugin-api/`](https://github.com/backstage/backstage/tree/master/packages/backend-plugin-api) -
+  This package contains the core APIs that are used to make Backstage backend features such as plugins and modules.
 
 - [`catalog-client`](https://github.com/backstage/backstage/tree/master/packages/catalog-client) -
   An isomorphic client to interact with the Software Catalog. Backend plugins