patch 8.2.4193: cannot use an import in 'charconvert'

Problem:    Cannot use an import in 'charconvert'.
Solution:   Set the script context when evaluating 'charconvert'.  Also expand
            script-local functions in 'charconvert'.
diff --git a/src/evalvars.c b/src/evalvars.c
index 196d4d4..305ffe5 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -369,17 +369,25 @@
     char_u	*fname_to)
 {
     int		err = FALSE;
+    sctx_T	saved_sctx = current_sctx;
+    sctx_T	*ctx;
 
     set_vim_var_string(VV_CC_FROM, enc_from, -1);
     set_vim_var_string(VV_CC_TO, enc_to, -1);
     set_vim_var_string(VV_FNAME_IN, fname_from, -1);
     set_vim_var_string(VV_FNAME_OUT, fname_to, -1);
+    ctx = get_option_sctx("charconvert");
+    if (ctx != NULL)
+	current_sctx = *ctx;
+
     if (eval_to_bool(p_ccv, &err, NULL, FALSE))
 	err = TRUE;
+
     set_vim_var_string(VV_CC_FROM, NULL, -1);
     set_vim_var_string(VV_CC_TO, NULL, -1);
     set_vim_var_string(VV_FNAME_IN, NULL, -1);
     set_vim_var_string(VV_FNAME_OUT, NULL, -1);
+    current_sctx = saved_sctx;
 
     if (err)
 	return FAIL;