cmd/go: clean paths before using them form index functions

We use str.TrimFilePathPrefix to trim the module root prefix and get the
relative path of each package in the module when scanning the module
and in the RelPath function.  Make sure to clean the path before
indexing and in RelPath to ensure that each path starts with that
prefix, because walk will clean the root path before joining each
subdirectory path to it.

Change-Id: I1dc1eddbd42030eb6d5d8e76a8675f94216447c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/411118
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/modindex/read.go

@@ -112,6 +112,7 @@ func Get(modroot string) (*ModuleIndex, error) {
 	if modroot == "" {
 		panic("modindex.Get called with empty modroot")
 	}
+	modroot = filepath.Clean(modroot)
 	isModCache := str.HasFilePathPrefix(modroot, cfg.GOMODCACHE)
 	return openIndex(modroot, isModCache)
 }
@@ -217,7 +218,7 @@ func (mi *ModuleIndex) Packages() []string {
 
 // RelPath returns the path relative to the module's root.
 func (mi *ModuleIndex) RelPath(path string) string {
-	return str.TrimFilePathPrefix(path, mi.modroot)
+	return str.TrimFilePathPrefix(filepath.Clean(path), mi.modroot) // mi.modroot is already clean
 }
 
 // ImportPackage is the equivalent of build.Import given the information in ModuleIndex.

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

@@ -0,0 +1,37 @@
+# Test a replacement with an absolute path (so the path isn't
+# cleaned by having filepath.Abs called on it). This checks
+# whether the modindex logic cleans the modroot path before using
+# it.
+
+[!windows] [short] skip
+
+go run print_go_mod.go # use this program to write a go.mod with an absolute path
+cp stdout go.mod
+
+go list -modfile=go.mod all
+-- print_go_mod.go --
+//go:build ignore
+package main
+
+import (
+    "fmt"
+    "os"
+)
+
+func main() {
+    work := os.Getenv("WORK")
+fmt.Printf(`module example.com/mod
+
+require b.com v0.0.0
+
+replace b.com => %s\gopath\src/modb
+`,  work)
+}
+-- a.go --
+package a
+
+import _ "b.com/b"
+-- modb/go.mod --
+module b.com
+-- modb/b/b.go --
+package b