Replace hex_erl_tar fork with OTP master's erl_tar (#165)

* Replace hex_erl_tar fork with OTP master's erl_tar

Copy erl_tar.erl and erl_tar.hrl from OTP commit 013041bd68c2 with
minimal modifications (module rename, include path, remove -doc/-moduledoc
attributes for OTP 24 compatibility).

This eliminates divergence from upstream. The fork's add/5 is replaced
with OTP's add/4 using the {mode, Mode} option. hex_filename.erl is
removed since filelib:safe_relative_path/2 is available since OTP 23.

* Fix erl_tar rejecting safe symlinks with .. in target

Join symlink targets with the symlink's parent directory before
validating. Previously the raw target was validated in isolation,
causing safe symlinks like dir/link -> ../file to be incorrectly
rejected as unsafe.

* Exclude generated files from formatting

Author: ericmj

rebar.config

@@ -27,7 +27,7 @@
 {erlfmt, [
     write,
     {files, ["{src,test,examples}/*.{hrl,erl,app.src}", "rebar.config"]},
-    {exclude_files, ["src/hex_pb_*.erl", "src/hex_erl_tar.{hrl,erl}"]}
+    {exclude_files, ["src/hex_pb_*.erl", "src/hex_erl_tar.{hrl,erl}", "src/safe_erl_term.erl"]}
 ]}.
 
 {profiles, [