Previously, `NgPackagesInstaller` would only look for Angular local
packages and do so by listing all (deeply nested) files in
`dist/packages-dist/` and looking for `package.json` files nested two
levels deep (i.e. `dist/packages-dist/*/package.json`). Thus, it would
unnecessarily check a large number of files.
This commit changes the package detection logic to instead look for
a `package.json` file directly inside each subdirectory of
`dist/packages-dist/`, which speeds up the operation.
It also refactors the code to make it easier to look for packages in
other directories (besides `dist/packages-dist/`). This will be useful
in a subsequent commit to look for and use the locally built `zone.js`
package (from `dist/zone.js-dist/`).
PR Close#35780
4c7d833233