… without a global CLI install

If the user does not have a global install of the Angular CLI, the autocompletion prompt is skipped and `ng completion` emits a warning. The reasoning for this is that `source <(ng completion script)` won't work without `ng` on the `$PATH`, which is only really viable with a global install. Local executions like `git clone ... && npm install && npm start` or ephemeral executions like `npx @angular/cli` don't benefit from autocompletion and unnecessarily impede users.

A global install of the Angular CLI is detected by running `which -a ng`, which appears to be a cross-platform means of listing all `ng` commands on the `$PATH`. We then look over all binaries in the list and exclude anything which is a directo child of a `node_modules/.bin/` directory. These include local executions and `npx`, so the only remaining locations should be global installs (`/usr/bin/ng`, NVM, etc.).

The tests are a little awkward since `ng` is installed globally by helper functions before tests start. These tests uninstall the global CLI and install a local, project-specific version to verify behavior, before restoring the global version. Hypothetically this could be emulated by manipulating the `$PATH` variable, but `which` needs to be available (so we can't clobber the whole `$PATH`) and `node` exists in the same directory as the global `ng` command (so we can't remove that directory anyways). There's also no good way of testing the case where `which` fails to run.

Closes angular#23135.