[PP-1] Package Manager's settings form should allow you to auto-detect the locations of Composer and rsync

So, this MR currently implements an explicit button for opting into auto-detection and clearing stored values:

When you click it, auto-detected executables are filled in, and ones that are not are left empty -- and you have to fill them in, since they are required fields (and you get a warning for the ones which were not found):

This has test coverage and needs usability review.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Fixed the merge conflict.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

Tentatively closing as won't fix, for the reasons explained in #3511972-70: Allow Composer and rsync location to be configured via the UI → .