Prevent copying or symlink non-existing src_dirs

This would cause crashes on linux and force people to have a src_dirs
config that is strictly matching what is on the file system rather than
acting as a specification of those that are valid.

To compare, if lib_dirs worked the same, then any repo that did not both
have apps/ and lib/ would crash, as the spec mentions both options as
valid.

src/rebar_prv_compile.erl

@@ -236,7 +236,10 @@ copy_app_dirs(AppInfo, OldAppDir, AppDir) ->
 symlink_or_copy(OldAppDir, AppDir, Dir) ->
     Source = filename:join([OldAppDir, Dir]),
     Target = filename:join([AppDir, Dir]),
-    rebar_file_utils:symlink_or_copy(Source, Target).
+    case ec_file:is_dir(Source) of
+        true -> rebar_file_utils:symlink_or_copy(Source, Target);
+        false -> ok
+    end.
 
 copy(OldAppDir, AppDir, Dir) ->
     Source = filename:join([OldAppDir, Dir]),

test/rebar_dir_SUITE.erl

@@ -79,13 +79,10 @@ alt_src_dir_nested(Config) ->
     RebarConfig = [{src_dirs, ["src", "alt/nested"]}],
     AppsDir = ?config(apps, Config),
     Name1 = ?config(app_one, Config),
-    Name2 = ?config(app_two, Config),
     ModDir = filename:join([AppsDir, "apps", Name1, "alt", "nested"]),
-    ModDir2 = filename:join([AppsDir, "apps", Name2, "alt", "nested"]),
     Mod = "-module(altmod). -export([main/0]). main() -> ok.",
 
     ec_file:mkdir_path(ModDir),
-    ec_file:mkdir_path(ModDir2),
     ok = file:write_file(filename:join([ModDir, "altmod.erl"]), Mod),
 
     Ebin = filename:join([AppsDir, "_build", "default", "lib", Name1, "ebin", "altmod.beam"]),