Add integration with project.el

This code is already used by eglot-jl to locate Julia projects and
will be useful in the future for other Julia packages (such as for
REPLs to decide which files they're associated with). Putting it in
the julia-mode package means it doesn't have to be redefined each time
it's used.

julia-mode.el

@@ -38,6 +38,7 @@
 
 (require 'cl-lib)
 (require 'julia-mode-latexsubs)
+(require 'project nil t)
 
 (defvar julia-mode-hook nil)
 
@@ -758,7 +759,14 @@ Return nil if point is not in a function, otherwise point."
   (setq-local end-of-defun-function #'julia-end-of-defun)
   (setq indent-tabs-mode nil)
   (setq imenu-generic-expression julia-imenu-generic-expression)
-  (imenu-add-to-menubar "Imenu"))
+  (imenu-add-to-menubar "Imenu")
+  ;; project.el integration
+  (when (featurep 'project)
+    (add-hook 'project-find-functions
+              (lambda (dir)
+                (let ((root (or (locate-dominating-file dir "JuliaProject.toml")
+                                (locate-dominating-file dir "Project.toml"))))
+                  (when root (cons 'julia root)))))))
 
 (defun julia-manual-deindent ()
   "Deindent by `julia-indent-offset' regardless of current
@@ -819,6 +827,19 @@ following commands are defined:
       (if julia-math-mode
           (setq-local LaTeX-math-insert-function #'julia-math-insert)))))
 
+;;; project.el integration
+;; project.el was added to emacs in 25.1, so we can't assume it's available
+(when (featurep 'project)
+  (defun julia--project-try (dir)
+    "Return project instance if DIR is part of a Julia project.
+Otherwise return nil."
+    (let ((root (or (locate-dominating-file dir "JuliaProject.toml")
+                    (locate-dominating-file dir "Project.toml"))))
+      (when root (cons 'julia root))))
+
+  (cl-defmethod project-roots ((project (head julia)))
+    (list (cdr project))))
+
 ;; Code for `inferior-julia-mode'
 (require 'comint)