Don't follow symlinks when adding files to tarballs

[fhunleth]

Path.wildcard("**") follows symlinks when collecting all of the files
under a path. This can lead to files in the tarball containing paths
with the symlink in them. If the directory corresponding to the symlink
hasn't been created, then the extraction will fail.

As an example, the "create with files" unit test now contains a symlink
in a directory. Without the fix, it fails like this:

  1) test create with files (Mix.Tasks.Hex.BuildTest)
     test/mix/tasks/hex.build_test.exs:42
     ** (MatchError) no match of right hand side value: {:error, :eexist}
     code: in_tmp(fn ->
     stacktrace:
       test/mix/tasks/hex.build_test.exs:13: Mix.Tasks.Hex.BuildTest.extract/2
       test/mix/tasks/hex.build_test.exs:69: anonymous fn/0 in Mix.Tasks.Hex.BuildTest."test create with files"/1
       (elixir) lib/file.ex:1443: File.cd!/2
       test/mix/tasks/hex.build_test.exs:45: (test)

Untaring the contents.tar.gz shows the problem.

$ tar tfz contents.tar.gz
myfile.txt
executable.sh
dir/.dotfile
dir/a_link_to_dir2
dir/a_link_to_dir2/test.txt
dir/dir2/test.txt
empty_dir/
link_dir

dir/a_link_to_dir2 is created as a symlink to dir/dir2. The
test.txt file is then extracted to it. This fails since dir2 hasn't
been created yet so a_link_to_dir2 is dangling. It's also not
desirable that test.txt was included twice.

This test seems like it is dependent on the order that the OS lists
files in a directory. My tests are on Linux, but OSX orders files
differently. The symlink starts with an "a" to try to force it to list
first.

After the fix in this commit, the contents.tar.gz looks like this:

$ tar tfz contents.tar.gz
myfile.txt
executable.sh
dir/a_link_to_dir2
dir/dir2/test.txt
dir/.dotfile
empty_dir/
link_dir

As you can see, test.txt is listed once and its parent directory is
the real directory, dir2, and not the symlink.

Fixes #631.

[wojtekmach]

Thank you!