go/packages: use cfg.abs(dir) in determineRootDirsModules

As determineRootDirsModules calls go list with a custom working
directory, relative paths that it produces should also be treated
relative to that directory (cfg.Dir).

Change-Id: I07e162a9119a4d36162e69702150e63acf96f207
Reviewed-on: https://go-review.googlesource.com/c/tools/+/680819
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>

Author: mateusz834

go/packages/golist_overlay.go

@@ -55,7 +55,7 @@ func (state *golistState) determineRootDirsModules() (map[string]string, error)
 		}
 		if mod.Dir != "" && mod.Path != "" {
 			// This is a valid module; add it to the map.
-			absDir, err := filepath.Abs(mod.Dir)
+			absDir, err := state.cfg.abs(mod.Dir)
 			if err != nil {
 				return nil, err
 			}