Enhance go project detection by referring to go.mod file (#1418)

This file is present in many (but not all) go projects. When go 1.13 comes out
later this year, the go.mod file might become a sufficient check for a go
project but today that is not the case.

CHANGELOG.md

@@ -8,6 +8,7 @@
 * [#982](https://github.com/bbatsov/projectile/issues/982) Add heuristic for projectile-find-matching-test
 * Support a list of functions for `related-files-fn` options and helper functions
 * [#1405](https://github.com/bbatsov/projectile/pull/1405) Add Bloop Scala build server project detection
+* [#1418](https://github.com/bbatsov/projectile/pull/1418) The presence of a go.mod file implies a go project
 * [#1419](https://github.com/bbatsov/projectile/pull/1419) When possible, use [fd](https://github.com/sharkdp/fd) instead
   of `find` to list the files of a non-VCS project. This should be much faster.
 

projectile.el

@@ -2508,7 +2508,8 @@ test/impl/other files as below:
 
 (defun projectile-go-project-p ()
   "Check if a project contains Go source files."
-  (projectile-verify-file-wildcard "*.go"))
+  (or (projectile-verify-file "go.mod")
+      (projectile-verify-file-wildcard "*.go")))
 
 (define-obsolete-variable-alias 'projectile-go-function 'projectile-go-project-test-function "1.0.0")
 (defcustom projectile-go-project-test-function #'projectile-go-project-p