caddyhttp: Add split_path to file matcher (used by php_fastcgi) (#3302)

* matcher: Add `split_path` option to file matcher; used in php_fastcgi

* matcher: Skip try_files split if not the final part of the filename

* matcher: Add MatchFile tests

* matcher: Clarify SplitPath godoc

modules/caddyhttp/reverseproxy/fastcgi/caddyfile.go

@@ -149,6 +149,7 @@ func parsePHPFastCGI(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error
 	rewriteMatcherSet := caddy.ModuleMap{
 		"file": h.JSON(fileserver.MatchFile{
 			TryFiles: []string{"{http.request.uri.path}", "{http.request.uri.path}/index.php", "index.php"},
+			SplitPath: []string{".php"},
 		}),
 	}
 	rewriteHandler := rewrite.Rewrite{