Rename "search path" to "library roots"

* lisp/emacs-lisp/cl-seq.el (cl-set-difference): Retain the order
of the elements from CL-LIST1.

* test/automated/cl-lib-tests.el (cl-lib-test-set-functions):
Update WRT to the above change.

* lisp/progmodes/project.el (project-search-path-function): Rename
to project-library-roots-function, update the documentation and
references.
(project-search-path): Likewise, to project-library-roots.
(project-roots): Clarify documentation.
(project-vc-search-path): Likewise, to project-vc-library-roots.
(project-library-roots): In addition to the renames, thread the
results through file-name-as-directory.
(project-prune-directories): Accept a variable number of
arguments.  Rename to project-combine-directories.
(project-subtract-directories): New function.

* lisp/progmodes/elisp-mode.el (elisp--xref-find-references):
Append project-roots and project-library-roots together.

* lisp/progmodes/etags.el (etags--xref-find-references): Ditto.

1 files changed, 3 insertions, 4 deletions

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index a222533936c..360d1baf403 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -802,10 +802,9 @@ to search in, and the file name pattern to search for."
          (dirs (if current-prefix-arg
                    (list (read-directory-name "Base directory: "
                                               nil default-directory t))
-                 (project-prune-directories
-                  (append
-                   (project-roots proj)
-                   (project-search-path proj)))))
+                 (append
+                  (project-roots proj)
+                  (project-library-roots proj))))
          (xref-find-function
           (lambda (_kind regexp)
             (cl-mapcan