makeWrapper: Only wrap normal executable files

`makeWrapper` and `wrapProgram` are being invoked on all kinds of
wacky things (usually with the help of bash globs or other machine
assistance).

So far, I have come across `wrapProgram` being invoked on a directory,
as well as on the empty string.

As far as I can tell, it's only valid to invoke these utilities on a
normal (non-directory, non-device) executable file.  This commit
enforces that precondition.

pkgs/build-support/setup-hooks/make-wrapper.sh

@@ -1,3 +1,12 @@
+# Assert that FILE exists and is executable
+#
+# assertExecutable FILE
+assertExecutable() {
+    local file="$1"
+    [[ -f "${file}" && -x "${file}" ]] || \
+        die "Cannot wrap ${file} because it is not an executable file"
+}
+
 # construct an executable file that wraps the actual executable
 # makeWrapper EXECUTABLE ARGS
 
@@ -24,6 +33,8 @@ makeWrapper() {
     local params varName value command separator n fileNames
     local argv0 flagsBefore flags
 
+    assertExecutable "${file}"
+
     mkdir -p "$(dirname "$wrapper")"
 
     echo "#! $SHELL -e" > "$wrapper"
@@ -131,6 +142,9 @@ filterExisting() {
 wrapProgram() {
     local prog="$1"
     local hidden
+
+    assertExecutable "${prog}"
+
     hidden="$(dirname "$prog")/.$(basename "$prog")"-wrapped
     while [ -e "$hidden" ]; do
       hidden="${hidden}_"