patch 8.2.3873: go.mod files are not recognized

Problem:    go.mod files are not recognized.
Solution:   Check for the file name. (closes #9380)

runtime/filetype.vim

@@ -1091,7 +1091,9 @@ au BufNewFile,BufRead *.mmp			setf mmp
 
 " Modsim III (or LambdaProlog)
 au BufNewFile,BufRead *.mod
-	\ if getline(1) =~ '\<module\>' |
+	\ if expand("<afile>") =~ '\<go.mod$' |
+	\   setf gomod |
+	\ elseif getline(1) =~ '\<module\>' |
 	\   setf lprolog |
 	\ else |
 	\   setf modsim3 |

src/testdir/test_filetype.vim

@@ -204,6 +204,7 @@ let s:filename_checks = {
     \ 'gnash': ['gnashrc', '.gnashrc', 'gnashpluginrc', '.gnashpluginrc'],
     \ 'gnuplot': ['file.gpi'],
     \ 'go': ['file.go'],
+    \ 'gomod': ['go.mod'],
     \ 'gp': ['file.gp', '.gprc'],
     \ 'gpg': ['/.gnupg/options', '/.gnupg/gpg.conf', '/usr/any/gnupg/options.skel', 'any/.gnupg/gpg.conf', 'any/.gnupg/options', 'any/usr/any/gnupg/options.skel'],
     \ 'grads': ['file.gs'],

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3873,
 /**/
     3872,
 /**/