Code maintenance: OS package for file system

- OS custom internal package for file system interaction
- Remove fileManager external dependency
- Closer API to Go's native API on the OS
- Create directories at startup
- Better testability
- Move Unsetenv to os interface

internal/os/os.go

@@ -0,0 +1,39 @@
+package os
+
+import nativeos "os"
+
+//go:generate mockgen -destination=mock_$GOPACKAGE/$GOFILE . OS
+
+type OS interface {
+	OpenFile(name string, flag int, perm FileMode) (File, error)
+	MkdirAll(name string, perm FileMode) error
+	Remove(name string) error
+	Chown(name string, uid int, gid int) error
+	Unsetenv(key string) error
+	Stat(name string) (nativeos.FileInfo, error)
+}
+
+func New() OS {
+	return &os{}
+}
+
+type os struct{}
+
+func (o *os) OpenFile(name string, flag int, perm FileMode) (File, error) {
+	return nativeos.OpenFile(name, flag, nativeos.FileMode(perm))
+}
+func (o *os) MkdirAll(name string, perm FileMode) error {
+	return nativeos.MkdirAll(name, nativeos.FileMode(perm))
+}
+func (o *os) Remove(name string) error {
+	return nativeos.Remove(name)
+}
+func (o *os) Chown(name string, uid, gid int) error {
+	return nativeos.Chown(name, uid, gid)
+}
+func (o *os) Unsetenv(key string) error {
+	return nativeos.Unsetenv(key)
+}
+func (o *os) Stat(name string) (nativeos.FileInfo, error) {
+	return nativeos.Stat(name)
+}