cmd/go: in 'go list -m', print effective go.mod file

When the -modfile flag is in use (either explicitly or from GOFLAGS),
'go list -m' will now print the effective go.mod file for the main
module in the GoMod field in -f or -json output.

Fixes #36220
Updates #34506

Change-Id: I89c2ee40f20e07854bb37c6e4e13eeea0cce7b0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/212100
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/list/list.go

@@ -211,7 +211,7 @@ applied to a Go struct, but now a Module struct:
         Main      bool         // is this the main module?
         Indirect  bool         // is this module only an indirect dependency of main module?
         Dir       string       // directory holding files for this module, if any
-        GoMod     string       // path to go.mod file for this module, if any
+        GoMod     string       // path to go.mod file used when loading this module, if any
         GoVersion string       // go version used in module
         Error     *ModuleError // error loading module
     }
@@ -220,6 +220,9 @@ applied to a Go struct, but now a Module struct:
         Err string // the error itself
     }
 
+The file GoMod refers to may be outside the module directory if the
+module is in the module cache or if the -modfile flag is used.
+
 The default output is to print the module path and then
 information about the version and replacement if any.
 For example, 'go list -m all' might print:

src/cmd/go/internal/modload/build.go

@@ -112,7 +112,7 @@ func moduleInfo(m module.Version, fromBuildList bool) *modinfo.ModulePublic {
 		}
 		if HasModRoot() {
 			info.Dir = ModRoot()
-			info.GoMod = filepath.Join(info.Dir, "go.mod")
+			info.GoMod = ModFilePath()
 			if modFile.Go != nil {
 				info.GoVersion = modFile.Go.Version
 			}

src/cmd/go/testdata/script/modfile_flag.txt

@@ -11,6 +11,15 @@ cp go.sum go.sum.orig
 go mod init example.com/m
 grep example.com/m go.alt.mod
 
+# 'go env GOMOD' should print the path to the real file.
+# 'go env' does not recognize the '-modfile' flag.
+go env GOMOD
+stdout '^\$WORK[/\\]gopath[/\\]src[/\\]go.mod$'
+
+# 'go list -m' should print the effective go.mod file as GoMod though.
+go list -m -f '{{.GoMod}}'
+stdout '^go.alt.mod$'
+
 # go mod edit should operate on the alternate file
 go mod edit -require rsc.io/quote@v1.5.2
 grep rsc.io/quote go.alt.mod