use module name with `module source`

The `module source` command only accepts file path as argument. It would be nice to allow the use of module name and version designation also on `module source` command.

```console
$ module source foo/1.2
```

As a consequence, to designate a file path instead of a module name and version, argument should be prefixed by `./`.

```console
$ module source ./dir/file
```