refactor: Reshuffle file structure for better code layout

This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.

This is the first step towards factoring out CLI-based functionality
for Nixery.

tools/nixery/logs/logs.go

@@ -0,0 +1,119 @@
+// Copyright 2019 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+package logs
+
+// This file configures different log formatters via logrus. The
+// standard formatter uses a structured JSON format that is compatible
+// with Stackdriver Error Reporting.
+//
+// https://cloud.google.com/error-reporting/docs/formatting-error-messages
+
+import (
+	"bytes"
+	"encoding/json"
+	log "github.com/sirupsen/logrus"
+)
+
+type stackdriverFormatter struct{}
+
+type serviceContext struct {
+	Service string `json:"service"`
+	Version string `json:"version"`
+}
+
+type reportLocation struct {
+	FilePath     string `json:"filePath"`
+	LineNumber   int    `json:"lineNumber"`
+	FunctionName string `json:"functionName"`
+}
+
+var nixeryContext = serviceContext{
+	Service: "nixery",
+}
+
+// isError determines whether an entry should be logged as an error
+// (i.e. with attached `context`).
+//
+// This requires the caller information to be present on the log
+// entry, as stacktraces are not available currently.
+func isError(e *log.Entry) bool {
+	l := e.Level
+	return (l == log.ErrorLevel || l == log.FatalLevel || l == log.PanicLevel) &&
+		e.HasCaller()
+}
+
+// logSeverity formats the entry's severity into a format compatible
+// with Stackdriver Logging.
+//
+// The two formats that are being mapped do not have an equivalent set
+// of severities/levels, so the mapping is somewhat arbitrary for a
+// handful of them.
+//
+// https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry#LogSeverity
+func logSeverity(l log.Level) string {
+	switch l {
+	case log.TraceLevel:
+		return "DEBUG"
+	case log.DebugLevel:
+		return "DEBUG"
+	case log.InfoLevel:
+		return "INFO"
+	case log.WarnLevel:
+		return "WARNING"
+	case log.ErrorLevel:
+		return "ERROR"
+	case log.FatalLevel:
+		return "CRITICAL"
+	case log.PanicLevel:
+		return "EMERGENCY"
+	default:
+		return "DEFAULT"
+	}
+}
+
+func (f stackdriverFormatter) Format(e *log.Entry) ([]byte, error) {
+	msg := e.Data
+	msg["serviceContext"] = &nixeryContext
+	msg["message"] = &e.Message
+	msg["eventTime"] = &e.Time
+	msg["severity"] = logSeverity(e.Level)
+
+	if e, ok := msg[log.ErrorKey]; ok {
+		if err, isError := e.(error); isError {
+			msg[log.ErrorKey] = err.Error()
+		} else {
+			delete(msg, log.ErrorKey)
+		}
+	}
+
+	if isError(e) {
+		loc := reportLocation{
+			FilePath:     e.Caller.File,
+			LineNumber:   e.Caller.Line,
+			FunctionName: e.Caller.Function,
+		}
+		msg["context"] = &loc
+	}
+
+	b := new(bytes.Buffer)
+	err := json.NewEncoder(b).Encode(&msg)
+
+	return b.Bytes(), err
+}
+
+func Init(version string) {
+	nixeryContext.Version = version
+	log.SetReportCaller(true)
+	log.SetFormatter(stackdriverFormatter{})
+}