build: include built-in XBlock JS directly rather than copying it (#32480)

As part of the static asset build, JS modules for most built-in XBlocks were
unnecessarily copied from the original locations (under xmodule/js and
common/static/js) to a git-ignored location (under common/static/xmodule), and
then included into the Webpack builld via
common/static/xmodule/webpack.xmodule.config.js.

With this commit, we stop copying the JS modules. Instead, we have
common/static/xmodule/webpack.xmodule.config.js just reference the original
source under xmodule/js and common/static/js.

This lets us us radically simplify the xmodule/static_content.py build script.
It also sets the stage for the next change, in which we will check
webpack.xmodule.config.js into the repository, and delete
xmodule/static_content.py entirely.

common/static/xmodule/webpack.xmodule.config.js before:

    module.exports = {
        "entry": {
            "AboutBlockDisplay": [
                "./common/static/xmodule/modules/js/000-b82f6c436159f6bc7ca2513e29e82503.js",
                "./common/static/xmodule/modules/js/001-3ed86006526f75d6c844739193a84c11.js",
                "./common/static/xmodule/modules/js/002-3918b2d4f383c04fed8227cc9f523d6e.js",
                "./common/static/xmodule/modules/js/003-b3206f2283964743c4772b9d72c67d64.js",
                "./common/static/xmodule/modules/js/004-274b8109ca3426c2a6fde9ec2c56e969.js",
                "./common/static/xmodule/modules/js/005-26caba6f71877f63a7dd4f6796109bf6.js"
            ],
            "AboutBlockEditor": [
                "./common/static/xmodule/descriptors/js/000-b82f6c436159f6bc7ca2513e29e82503.js",
                "./common/static/xmodule/descriptors/js/001-19c4723cecaa5a5a46b8566b3544e732.js"
            ],
            // etc
        }
    };

common/static/xmodule/webpack.xmodule.config.js after:

    module.exports = {
        "entry": {
            "AboutBlockDisplay": [
                "./xmodule/js/src/xmodule.js",
                "./xmodule/js/src/html/display.js",
                "./xmodule/js/src/javascript_loader.js",
                "./xmodule/js/src/collapsible.js",
                "./xmodule/js/src/html/imageModal.js",
                "./xmodule/js/common_static/js/vendor/draggabilly.js"
            ],
            "AboutBlockEditor": [
                "./xmodule/js/src/xmodule.js",
                "./xmodule/js/src/html/edit.js"
            ],
            // etc
        }
    };

Part of: https://github.com/openedx/edx-platform/issues/32481

conf/locale/config.yaml

@@ -98,8 +98,6 @@ dummy_locales:
 
 # Directories we don't search for strings.
 ignore_dirs:
-    - common/static/xmodule/modules
-    - common/static/xmodule/descriptors
     # Directories with no user-facing code.
     - '*/migrations'
     - '*/envs'