move some packages to the "internal" directory

- this makes it impossible to import these packages outside Task
- as a side effect, it makes the root directory cleaner

internal/execext/exec.go

@@ -0,0 +1,56 @@
+package execext
+
+import (
+	"context"
+	"errors"
+	"io"
+	"strings"
+
+	"mvdan.cc/sh/interp"
+	"mvdan.cc/sh/syntax"
+)
+
+// RunCommandOptions is the options for the RunCommand func
+type RunCommandOptions struct {
+	Context context.Context
+	Command string
+	Dir     string
+	Env     []string
+	Stdin   io.Reader
+	Stdout  io.Writer
+	Stderr  io.Writer
+}
+
+var (
+	// ErrNilOptions is returned when a nil options is given
+	ErrNilOptions = errors.New("execext: nil options given")
+)
+
+// RunCommand runs a shell command
+func RunCommand(opts *RunCommandOptions) error {
+	if opts == nil {
+		return ErrNilOptions
+	}
+
+	p, err := syntax.NewParser().Parse(strings.NewReader(opts.Command), "")
+	if err != nil {
+		return err
+	}
+
+	r := interp.Runner{
+		Context: opts.Context,
+		Dir:     opts.Dir,
+		Env:     opts.Env,
+
+		Exec: interp.DefaultExec,
+		Open: interp.OpenDevImpls(interp.DefaultOpen),
+
+		Stdin:  opts.Stdin,
+		Stdout: opts.Stdout,
+		Stderr: opts.Stderr,
+	}
+	if err = r.Reset(); err != nil {
+		return err
+	}
+	return r.Run(p)
+}