gnu: Fix references to emacs-build-system's expand-load-path phase.

* gnu/packages/emacs-xyz.scm (emacs-pdf-tools)[#:phases]: Use
emacs-expand-load-path instead of emacs-add-source-to-load-path.
* gnu/packages/erlang.scm (emacs-erlang)[#:phases]: Use expand-load-path
instead of add-source-to-load-path.
* gnu/packages/protobuf.scm (emacs-protobuf-mode)[#:phases]: Use
expand-load-path instead of add-source-to-load-path.

Signed-off-by: Leo Prikler <leo.prikler@student.tugraz.at>

gnu/packages/emacs-xyz.scm

@@ -3310,7 +3310,7 @@ during idle time, while Emacs is doing nothing else.")
                  ("pdf-tools-handle-upgrades" '()))))
            (add-after 'emacs-patch-variables 'emacs-expand-load-path
              (assoc-ref emacs:%standard-phases 'expand-load-path))
-           (add-after 'emacs-add-source-to-load-path 'emacs-install
+           (add-after 'emacs-expand-load-path 'emacs-install
              (assoc-ref emacs:%standard-phases 'install))
            (add-after 'emacs-install 'emacs-build
              (assoc-ref emacs:%standard-phases 'build))

gnu/packages/erlang.scm

@@ -232,7 +232,7 @@ built-in support for concurrency, distribution and fault tolerance.")
     (arguments
      `(#:phases
        (modify-phases %standard-phases
-         (add-before 'add-source-to-load-path 'change-working-directory
+         (add-before 'expand-load-path 'change-working-directory
            (lambda _ (chdir "lib/tools/emacs") #t)))))
     (home-page "https://www.erlang.org/")
     (synopsis "Erlang major mode for Emacs")

gnu/packages/protobuf.scm

@@ -311,7 +311,7 @@ structured data.")
     (arguments
      `(#:phases
        (modify-phases %standard-phases
-         (add-before 'add-source-to-load-path 'change-working-directory
+         (add-before 'expand-load-path 'change-working-directory
            (lambda _ (chdir "editors") #t)))))
     (home-page "https://github.com/protocolbuffers/protobuf")
     (synopsis "Protocol buffers major mode for Emacs")