set entrypoints to use ./lib rather than ./src (matrix-org#4357)

Currently, we replace the entrypoints in package.json during the release
cycle. I think. historically, this was done to make matrix-react-sdk and
element-web development easier, but neither of those projects actually use
these entrypoints (instead they import from `src`).

Accordingly, I think the switcheroo is unnecessary; furthermore it causes a
whole bunch of confusion by making the development environment different from
the release environment, and it complicates our CI and release process.

In short, the switcheroo has to die.

package.json

@@ -32,13 +32,9 @@
         "matrix-org"
     ],
     "type": "module",
-    "main": "./src/index.ts",
-    "browser": "./src/browser-index.ts",
-    "matrix_src_main": "./src/index.ts",
-    "matrix_src_browser": "./src/browser-index.ts",
-    "matrix_lib_main": "./lib/index.js",
-    "matrix_lib_browser": "./lib/browser-index.js",
-    "matrix_lib_typings": "./lib/index.d.ts",
+    "main": "./lib/index.js",
+    "browser": "./lib/browser-index.js",
+    "typings": "./lib/index.d.ts",
     "author": "matrix.org",
     "license": "Apache-2.0",
     "files": [