Revert: keep standard package list

If no go in the PATH, packages will raise error:
```
err: go command required, not found: exec: "go": executable file not found in $PATH: stderr:
```

Signed-off-by: Loong <long0dai@foxmail.com>

Makefile

@@ -1,8 +1,8 @@
-.PHONY: clean test build
+.PHONY: clean generate test build
 
 BIN_OUTPUT := $(if $(filter $(shell go env GOOS), windows), dist/gci.exe, dist/gci)
 
-default: clean test build
+default: clean generate test build
 
 clean:
 	@echo BIN_OUTPUT: ${BIN_OUTPUT}
@@ -13,3 +13,6 @@ build: clean
 
 test: clean
 	@go test -v -count=1 -cover ./...
+
+generate:
+	@go generate ./...

go.mod

@@ -1,6 +1,6 @@
 module github.com/daixiang0/gci
 
-go 1.20
+go 1.18
 
 require (
 	github.com/hexops/gotextdiff v1.0.3

pkg/section/parser.go

@@ -22,7 +22,7 @@ func Parse(data []string) (SectionList, error) {
 		if s == "default" {
 			list = append(list, Default{})
 		} else if s == "standard" {
-			list = append(list, NewStandard())
+			list = append(list, Standard{})
 		} else if s == "newline" {
 			list = append(list, NewLine{})
 		} else if strings.HasPrefix(s, "prefix(") && len(d) > 8 {

pkg/section/section.go

@@ -28,7 +28,7 @@ func (list SectionList) String() []string {
 }
 
 func DefaultSections() SectionList {
-	return SectionList{NewStandard(), Default{}}
+	return SectionList{Standard{}, Default{}}
 }
 
 func DefaultSectionSeparators() SectionList {

pkg/section/standard.go

@@ -1,33 +1,16 @@
 package section
 
 import (
-	"golang.org/x/tools/go/packages"
-
 	"github.com/daixiang0/gci/pkg/parse"
 	"github.com/daixiang0/gci/pkg/specificity"
 )
 
 const StandardType = "standard"
 
-type Standard struct {
-	standardPackages map[string]struct{}
-}
-
-func NewStandard() Standard {
-	pkgs, err := packages.Load(nil, "std")
-	if err != nil {
-		panic(err)
-	}
-
-	standardPackages := make(map[string]struct{})
-	for _, p := range pkgs {
-		standardPackages[p.PkgPath] = struct{}{}
-	}
-	return Standard{standardPackages: standardPackages}
-}
+type Standard struct{}
 
 func (s Standard) MatchSpecificity(spec *parse.GciImports) specificity.MatchSpecificity {
-	if _, ok := s.standardPackages[spec.Path]; ok {
+	if isStandard(spec.Path) {
 		return specificity.StandardMatch{}
 	}
 	return specificity.MisMatch{}
@@ -40,3 +23,8 @@ func (s Standard) String() string {
 func (s Standard) Type() string {
 	return StandardType
 }
+
+func isStandard(pkg string) bool {
+	_, ok := standardPackages[pkg]
+	return ok
+}

pkg/section/standard_test.go

@@ -7,15 +7,14 @@ import (
 )
 
 func TestStandardPackageSpecificity(t *testing.T) {
-	standard := NewStandard()
 	testCases := []specificityTestData{
-		{"context", standard, specificity.StandardMatch{}},
-		{"contexts", standard, specificity.MisMatch{}},
-		{"crypto", standard, specificity.StandardMatch{}},
-		{"crypto1", standard, specificity.MisMatch{}},
-		{"crypto/ae", standard, specificity.MisMatch{}},
-		{"crypto/aes", standard, specificity.StandardMatch{}},
-		{"crypto/aes2", standard, specificity.MisMatch{}},
+		{"context", Standard{}, specificity.StandardMatch{}},
+		{"contexts", Standard{}, specificity.MisMatch{}},
+		{"crypto", Standard{}, specificity.StandardMatch{}},
+		{"crypto1", Standard{}, specificity.MisMatch{}},
+		{"crypto/ae", Standard{}, specificity.MisMatch{}},
+		{"crypto/aes", Standard{}, specificity.StandardMatch{}},
+		{"crypto/aes2", Standard{}, specificity.MisMatch{}},
 	}
 	testSpecificity(t, testCases)
 }