Don't require relative paths to start with ./ or ../

Errors reported by Go, even when containing relative paths, do not
always begin with ./ or ../ – for example:

	$ go build ./...
	# honnef.co/go/tools/lintcmd
	lintcmd/format.go:28:8: syntax error: cannot use path := filepath.Clean(pos.Filename) as value

We could require either ./, ../ or at least one path separator and
still match Go's output. However, commonly used linters (such as
Staticcheck and golint) never use ./ for relative paths. Their output
stopped being matched when we moved from v1 to v2. I believe that
being able to match the output of linters is worth relaxing the
pattern for.

This change slightly relaxes the stricter pattern that was introduced
as part of v2 to address actions#46. However, the pattern is still stricter
than it was in v1 and as strict as it can be for most users.

__tests__/setup-go.test.ts

@@ -497,6 +497,16 @@ describe('setup-go', () => {
     expect(annotation.message).toBe('undefined: fmt.Printl');
   });
 
+  it('matches on unix path down the tree', async () => {
+    let line = 'foo/main.go:13:2: undefined: fmt.Printl';
+    let annotation = testMatch(line);
+    expect(annotation).toBeDefined();
+    expect(annotation.line).toBe(13);
+    expect(annotation.column).toBe(2);
+    expect(annotation.file).toBe('foo/main.go');
+    expect(annotation.message).toBe('undefined: fmt.Printl');
+  });
+
   it('matches on rooted unix path', async () => {
     let line = '/assert.go:4:1: missing return at end of function';
     let annotation = testMatch(line);

matchers.json

@@ -4,7 +4,7 @@
             "owner": "go",
             "pattern": [
                 {
-                    "regexp": "^\\s*(\\.{0,2}[\\/\\\\].+\\.go):(?:(\\d+):(\\d+):)? (.*)",
+                    "regexp": "^\\s*(.+\\.go):(?:(\\d+):(\\d+):)? (.*)",
                     "file": 1,
                     "line": 2,
                     "column": 3,
@@ -13,4 +13,4 @@
             ]
         }
     ]
-}
+}