Update megacheck to the latest version

Also do following improvements:
  - show proper sublinter name for megacheck sublinters
  - refactor and make more simple and robust megacheck
  merging/optimizing
  - improve handling of unknown linter names in //nolint directives
  - minimize diff of our megacheck version from the upstream,
  https://github.com/golang/go/issues/29612 blocks usage of the upstream
  version
  - support the new `stylecheck` linter
  - improve tests coverage for megacheck and nolint related cases
  - update and use upstream versions of unparam and interfacer instead of forked
  ones
  - don't use golangci/tools repo anymore
  - fix newly found issues after updating linters

Also should be noted that megacheck works much faster and consumes less
memory in the newest release, therefore golangci-lint works noticeably
faster and consumes less memory for large repos.

Relates: #314

test/enabled_linters_test.go

@@ -0,0 +1,189 @@
+package test
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+	"testing"
+
+	"github.com/golangci/golangci-lint/pkg/lint/lintersdb"
+	"github.com/golangci/golangci-lint/test/testshared"
+)
+
+func inSlice(s []string, v string) bool {
+	for _, sv := range s {
+		if sv == v {
+			return true
+		}
+	}
+
+	return false
+}
+
+func getEnabledByDefaultFastLintersExcept(except ...string) []string {
+	m := lintersdb.NewManager()
+	ebdl := m.GetAllEnabledByDefaultLinters()
+	ret := []string{}
+	for _, lc := range ebdl {
+		if lc.NeedsSSARepr {
+			continue
+		}
+
+		if !inSlice(except, lc.Name()) {
+			ret = append(ret, lc.Name())
+		}
+	}
+
+	return ret
+}
+
+func getAllFastLintersWith(with ...string) []string {
+	linters := lintersdb.NewManager().GetAllSupportedLinterConfigs()
+	ret := append([]string{}, with...)
+	for _, lc := range linters {
+		if lc.NeedsSSARepr {
+			continue
+		}
+		ret = append(ret, lc.Name())
+	}
+
+	return ret
+}
+
+func getEnabledByDefaultLinters() []string {
+	ebdl := lintersdb.NewManager().GetAllEnabledByDefaultLinters()
+	ret := []string{}
+	for _, lc := range ebdl {
+		ret = append(ret, lc.Name())
+	}
+
+	return ret
+}
+
+func getEnabledByDefaultFastLintersWith(with ...string) []string {
+	ebdl := lintersdb.NewManager().GetAllEnabledByDefaultLinters()
+	ret := append([]string{}, with...)
+	for _, lc := range ebdl {
+		if lc.NeedsSSARepr {
+			continue
+		}
+
+		ret = append(ret, lc.Name())
+	}
+
+	return ret
+}
+
+func TestEnabledLinters(t *testing.T) {
+	type tc struct {
+		name           string
+		cfg            string
+		el             []string
+		args           string
+		noImplicitFast bool
+	}
+
+	cases := []tc{
+		{
+			name: "disable govet in config",
+			cfg: `
+			linters:
+				disable:
+					- govet
+			`,
+			el: getEnabledByDefaultFastLintersExcept("govet"),
+		},
+		{
+			name: "enable golint in config",
+			cfg: `
+			linters:
+				enable:
+					- golint
+			`,
+			el: getEnabledByDefaultFastLintersWith("golint"),
+		},
+		{
+			name: "disable govet in cmd",
+			args: "-Dgovet",
+			el:   getEnabledByDefaultFastLintersExcept("govet"),
+		},
+		{
+			name: "enable gofmt in cmd and enable golint in config",
+			args: "-Egofmt",
+			cfg: `
+			linters:
+				enable:
+					- golint
+			`,
+			el: getEnabledByDefaultFastLintersWith("golint", "gofmt"),
+		},
+		{
+			name: "fast option in config",
+			cfg: `
+			linters:
+				fast: true
+			`,
+			el:             getEnabledByDefaultFastLintersWith(),
+			noImplicitFast: true,
+		},
+		{
+			name: "explicitly unset fast option in config",
+			cfg: `
+			linters:
+				fast: false
+			`,
+			el:             getEnabledByDefaultLinters(),
+			noImplicitFast: true,
+		},
+		{
+			name:           "set fast option in command-line",
+			args:           "--fast",
+			el:             getEnabledByDefaultFastLintersWith(),
+			noImplicitFast: true,
+		},
+		{
+			name: "fast option in command-line has higher priority to enable",
+			cfg: `
+			linters:
+				fast: false
+			`,
+			args:           "--fast",
+			el:             getEnabledByDefaultFastLintersWith(),
+			noImplicitFast: true,
+		},
+		{
+			name: "fast option in command-line has higher priority to disable",
+			cfg: `
+			linters:
+				fast: true
+			`,
+			args:           "--fast=false",
+			el:             getEnabledByDefaultLinters(),
+			noImplicitFast: true,
+		},
+		{
+			name:           "fast option combined with enable and enable-all",
+			args:           "--enable-all --fast --enable=staticcheck",
+			el:             getAllFastLintersWith("staticcheck"),
+			noImplicitFast: true,
+		},
+	}
+
+	for _, c := range cases {
+		c := c
+		t.Run(c.name, func(t *testing.T) {
+			runArgs := []string{"-v"}
+			if !c.noImplicitFast {
+				runArgs = append(runArgs, "--fast")
+			}
+			if c.args != "" {
+				runArgs = append(runArgs, strings.Split(c.args, " ")...)
+			}
+			r := testshared.NewLintRunner(t).RunWithYamlConfig(c.cfg, runArgs...)
+			sort.StringSlice(c.el).Sort()
+
+			expectedLine := fmt.Sprintf("Active %d linters: [%s]", len(c.el), strings.Join(c.el, " "))
+			r.ExpectOutputContains(expectedLine)
+		})
+	}
+}