Do use elpa deps if there are no el-get :depends

After #1798 a recipe like `(:name some-package :type elpa)` won't get the needed dependencies, even though elpa knows what they are. It's more convenient to elide `:depends`, so we let elpa download dependencies if the recipes fails to specify any.

Resolves #1951.

```
* methods/el-get-elpa.el (el-get-elpa-install-1-package): rename to
el-get-elpa-install-package, take another argument have-deps-p. Only
call package-download-transaction directly if have-deps-p.
(el-get-elpa-install, el-get-elpa-update): el-get-elpa-install{-1
=>}-package name change and new arg.
```