After the JavaScript reorganization in [43309], it was no longer possible to test WordPress from the `src` folder. That meant a build step was required to test PHP modifications. That is suboptimal as even a simple copy is slower than a web server just serving the new file.
We achieve building to `src` by setting a `WORKING_DIR` constant in the Gruntfile that is `build` by default, but changes to `src` when the `--dev` flag is present on any Grunt command. We provide sensible defaults so some commands, such as copying `version.php`, always build to `build`.
Because testing from `build` is no longer required, we change the messages present in `index.php` and `wp-admin/index.php` to be more broadly about building WordPress.
We also change the webpack config to have more straightforward behavior based on the `buildTarget` argument. It only determines the build target now and has no implicit behavior anymore. `grunt build` still works as it worked before, to make sure that the build server produces the same `wordpress.zip` we are used to.
We do all this instead of a symlink setup because symlinks don't work on every platform.
Props omarreiss, netweb, flixos90, SergeyBiryukov.
Fixes#44492.
Built from https://develop.svn.wordpress.org/trunk@44359
git-svn-id: http://core.svn.wordpress.org/trunk@44189 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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