Always tell elpa about el-get activated packages

Previously we would only tell elpa about :type elpa packages, but this
doesn't work if some of the dependencies of an elpa package are not
themselves elpa packages. Since we already require all package names to
be unique, there should be no risk in telling elpa about all el-get
packages.

Also, checking for el-get installed :type elpa packages is not needed,
since any :type elpa package implicitly depends on package, so we can
just check the feature.

```
* el-get.el (el-get-do-init): always add package symbol to
  package-activated-list when (featurep 'package).
```
---
Fixes flaw in #1798, resolves #1958.