This is my setup:

Mac OS          10.12.1
ViM             8.0.52
Exuberant Ctags 5.8

My .vimrc has: set tags=tags

I’ve created a ~/.ctags file with the following patterns from the Elixir cTags repo on github:

--langdef=Elixir
--langmap=Elixir:.ex.exs
--regex-Elixir=/^[ \t]*def(p?)[ \t]+([a-z_][a-zA-Z0-9_?!]*)/\2/f,functions,functions (def ...)/
--regex-Elixir=/^[ \t]*defcallback[ \t]+([a-z_][a-zA-Z0-9_?!]*)/\1/c,callbacks,callbacks (defcallback ...)/
--regex-Elixir=/^[ \t]*defdelegate[ \t]+([a-z_][a-zA-Z0-9_?!]*)/\1/d,delegates,delegates (defdelegate ...)/
--regex-Elixir=/^[ \t]*defexception[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/e,exceptions,exceptions (defexception ...)/
--regex-Elixir=/^[ \t]*defimpl[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/i,implementations,implementations (defimpl ...)/
--regex-Elixir=/^[ \t]*defmacro(p?)[ \t]+([a-z_][a-zA-Z0-9_?!]*)\(/\2/a,macros,macros (defmacro ...)/
--regex-Elixir=/^[ \t]*defmacro(p?)[ \t]+([a-zA-Z0-9_?!]+)?[ \t]+([^ \tA-Za-z0-9_]+)[ \t]*[a-zA-Z0-9_!?!]/\3/o,operators,operators (e.g. "defmacro a <<< b")/
--regex-Elixir=/^[ \t]*defmodule[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/m,modules,modules (defmodule ...)/
--regex-Elixir=/^[ \t]*defprotocol[ \t]+([A-Z][a-zA-Z0-9_]*\.)*([A-Z][a-zA-Z0-9_?!]*)/\2/p,protocols,protocols (defprotocol...)/
--regex-Elixir=/^[ \t]*Record\.defrecord[ \t]+:([a-zA-Z0-9_]+)/\1/r,records,records (defrecord...)/
--regex-Elixir=/^[ \t]*test[ \t]+\"([a-z_][a-zA-Z0-9_?! ]*)\"*/\1/t,tests,tests (test ...)/

The ctags navigation within my own project files work just fine. But whenever I try to see the source code for the Phoenix Framework itself (or its dependencies) I get a

E426: tag not found: Mix

(Mix is just one example)

How do I fix this and navigate Phoenix’s source code?