tools/linters: convert to LinterPlugin implementation

This commit introduces the `LLPlugin` type and converts the existing lll
code such that the LLPlugin implements the register.LinterPlugin
interface. This will allow us to plug it into golangci-linter as a
plugin.

tools/linters/ll_test.go

@@ -90,7 +90,9 @@ func TestGetLLLIssuesForFile(t *testing.T) {
 			require.Len(t, issues, len(tc.expectedIssue))
 
 			for i, issue := range issues {
-				require.Equal(t, tc.expectedIssue[i], issue.Text)
+				require.Equal(
+					t, tc.expectedIssue[i], issue.text,
+				)
 			}
 		})
 	}