add Go 1.20, drop 1.18

While here, clean up code to support older versions of Go.
Note that none of these changes should explicitly break Go 1.18.

.github/workflows/test.yml

@@ -12,8 +12,8 @@ jobs:
       fail-fast: false
       matrix:
         go-version:
-        - '1.18.x'
         - '1.19.x'
+        - '1.20.x'
         os:
         - ubuntu-latest
         - macos-11
@@ -32,7 +32,7 @@ jobs:
         go test -race ./...
 
     - name: Tidy
-      if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.19.x' # no need to do this everywhere
+      if: matrix.os == 'ubuntu-latest' && matrix.go-version == '1.20.x' # no need to do this everywhere
       run: |
         go mod tidy
 

cmd/testscript/testdata/env_var_with_go.txt

@@ -13,8 +13,7 @@ unquote withproxy.txt
 testscript -v noproxy.txt
 stdout ^BANANA=$
 stdout '^GOPATH=\$WORK[/\\]\.gopath'$
-[!go1.13] stdout ^GOPROXY=$
-[go1.13] stdout ^GOPROXY=https://proxy.golang.org,direct$
+stdout ^GOPROXY=https://proxy.golang.org,direct$
 ! stderr .+
 
 env BANANA=banana
@@ -26,8 +25,7 @@ env GOPROXY=
 testscript -v noproxy.txt
 stdout ^BANANA=$
 stdout '^GOPATH=\$WORK[/\\]\.gopath'$
-[!go1.13] stdout ^GOPROXY=$
-[go1.13] stdout ^GOPROXY=https://proxy.golang.org,direct$
+stdout ^GOPROXY=https://proxy.golang.org,direct$
 ! stderr .+
 
 # no GOPROXY, no pass-through, with proxy

fmtsort/mapelem.go

@@ -1,6 +1,3 @@
-//go:build go1.12
-// +build go1.12
-
 package fmtsort
 
 import "reflect"

go.mod

@@ -1,3 +1,3 @@
 module github.com/rogpeppe/go-internal
 
-go 1.18
+go 1.19

goproxytest/testdata/list.txt

@@ -1,6 +1,3 @@
-# prior to go 1.12 you cannot list a module without a requirement
-[!go1.12] go get fruit.com
-
 go list -m -versions fruit.com
 stdout 'v1.0.0 v1.1.0'
 

testscript/exe_go118.go

@@ -1,6 +1,3 @@
-//go:build go1.18
-// +build go1.18
-
 package testscript
 
 import (