Add self-contained gometalinter build tooling.

tools/vendor/github.com/alecthomas/gometalinter/aggregate.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"sort"
+	"strings"
+)
+
+type (
+	issueKey struct {
+		path      string
+		line, col int
+		message   string
+	}
+
+	multiIssue struct {
+		*Issue
+		linterNames []string
+	}
+)
+
+func aggregateIssues(issues chan *Issue) chan *Issue {
+	out := make(chan *Issue, 1000000)
+	issueMap := make(map[issueKey]*multiIssue)
+	go func() {
+		for issue := range issues {
+			key := issueKey{
+				path:    issue.Path,
+				line:    issue.Line,
+				col:     issue.Col,
+				message: issue.Message,
+			}
+			if existing, ok := issueMap[key]; ok {
+				existing.linterNames = append(existing.linterNames, issue.Linter.Name)
+			} else {
+				issueMap[key] = &multiIssue{
+					Issue:       issue,
+					linterNames: []string{issue.Linter.Name},
+				}
+			}
+		}
+		for _, multi := range issueMap {
+			issue := multi.Issue
+			sort.Strings(multi.linterNames)
+			issue.Linter.Name = strings.Join(multi.linterNames, ", ")
+			out <- issue
+		}
+		close(out)
+	}()
+	return out
+}