Expand environment variables on "dir", "sources" and "generates"

So a path like this works: $GOPATH/src/github.com/go-task/task

Allowing of "~" was also implemented. See #74 and baac067

Fixes #116

Taskfile.yml

@@ -13,6 +13,7 @@ vars:
     ./internal/compiler/v2
     ./internal/execext
     ./internal/logger
+    ./internal/osext
     ./internal/output
     ./internal/status
     ./internal/taskfile

internal/osext/osext.go

@@ -0,0 +1,22 @@
+package osext
+
+import (
+	"os"
+
+	"github.com/mitchellh/go-homedir"
+)
+
+// Expand is an improved version of os.ExpandEnv,
+// that not only expand enrionment variable ($GOPATH/src/github.com/...)
+// but also expands "~" as the home directory.
+func Expand(s string) (string, error) {
+	s = os.ExpandEnv(s)
+
+	var err error
+	s, err = homedir.Expand(s)
+	if err != nil {
+		return "", err
+	}
+
+	return s, nil
+}

internal/status/glob.go

@@ -4,16 +4,17 @@ import (
 	"path/filepath"
 	"sort"
 
+	"github.com/go-task/task/internal/osext"
+
 	"github.com/mattn/go-zglob"
-	"github.com/mitchellh/go-homedir"
 )
 
 func glob(dir string, globs []string) (files []string, err error) {
 	for _, g := range globs {
 		if !filepath.IsAbs(g) {
 			g = filepath.Join(dir, g)
 		}
-		g, err = homedir.Expand(g)
+		g, err = osext.Expand(g)
 		if err != nil {
 			return nil, err
 		}

variables.go

@@ -3,10 +3,9 @@ package task
 import (
 	"path/filepath"
 
+	"github.com/go-task/task/internal/osext"
 	"github.com/go-task/task/internal/taskfile"
 	"github.com/go-task/task/internal/templater"
-
-	"github.com/mitchellh/go-homedir"
 )
 
 var (
@@ -41,7 +40,7 @@ func (e *Executor) CompiledTask(call taskfile.Call) (*taskfile.Task, error) {
 		Method:    r.Replace(origTask.Method),
 		Prefix:    r.Replace(origTask.Prefix),
 	}
-	new.Dir, err = homedir.Expand(new.Dir)
+	new.Dir, err = osext.Expand(new.Dir)
 	if err != nil {
 		return nil, err
 	}