patch 8.1.2128: renamed libvterm sources makes merging difficult

Problem:    Renamed libvterm sources makes merging difficult.
Solution:   Rename back to the original name and only rename the .o files.
            Also clean the libvterm build artifacts. (James McCoy,
            closes #5027)
diff --git a/src/Make_cyg_ming.mak b/src/Make_cyg_ming.mak
index f04d11b..24c1338 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -869,15 +869,15 @@
 
 ifeq ($(TERMINAL),yes)
 OBJ += $(OUTDIR)/terminal.o \
-	$(OUTDIR)/encoding.o \
-	$(OUTDIR)/keyboard.o \
-	$(OUTDIR)/termmouse.o \
-	$(OUTDIR)/parser.o \
-	$(OUTDIR)/pen.o \
-	$(OUTDIR)/termscreen.o \
-	$(OUTDIR)/state.o \
-	$(OUTDIR)/unicode.o \
-	$(OUTDIR)/vterm.o
+	$(OUTDIR)/vterm_encoding.o \
+	$(OUTDIR)/vterm_keyboard.o \
+	$(OUTDIR)/vterm_mouse.o \
+	$(OUTDIR)/vterm_parser.o \
+	$(OUTDIR)/vterm_pen.o \
+	$(OUTDIR)/vterm_screen.o \
+	$(OUTDIR)/vterm_state.o \
+	$(OUTDIR)/vterm_unicode.o \
+	$(OUTDIR)/vterm_vterm.o
 endif
 
 ifeq ($(SOUND),yes)
@@ -1206,7 +1206,7 @@
 	  -DWCWIDTH_FUNCTION=utf_uint2cells \
 	  -DGET_SPECIAL_PTY_TYPE_FUNCTION=get_special_pty_type
 
-$(OUTDIR)/%.o : libvterm/src/%.c $(TERM_DEPS)
+$(OUTDIR)/vterm_%.o : libvterm/src/%.c $(TERM_DEPS)
 	$(CCCTERM) $< -o $@