Allow bare errors from golang.org/x/sys/unix

This is my naive (and incomplete) approach to fix GH issue #10.

The problems with this implementation are:
- no test cases (it seems those would need the whole golang.org/x/sys/unix
  to be available under testdata/src);
- a check is too relaxed (ignoring all errors that start from "E" from
  any package named "unix").

Alas, I have no idea how to fix those.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

errorlint/allowed.go

@@ -3,6 +3,7 @@ package errorlint
 import (
 	"fmt"
 	"go/ast"
+	"strings"
 )
 
 var allowedErrors = []struct {
@@ -101,11 +102,19 @@ func isAllowedErrorComparison(info *TypesInfoExt, binExpr *ast.BinaryExpr) bool
 		}
 	}
 
+	fmt.Printf("errName: %s\n", errName)
 	// Unimplemented or not sure, disallow the expression.
 	if errName == "" || len(callExprs) == 0 {
 		return false
 	}
 
+	// As an exception, allow all errors from golang.org/x/sys/unix as they are bare.
+	// Note this check is too relaxed because the package name is not complete.
+	// See https://github.com/polyfloyd/go-errorlint/issues/10.
+	if strings.HasPrefix(errName, "unix.E") {
+		return true
+	}
+
 	// Map call expressions to the function name format of the allow list.
 	functionNames := make([]string, len(callExprs))
 	for i, callExpr := range callExprs {