Read all recipes fixes

Make package list use merged recipe definitions

el-get-list-packages wasn't showing descriptions when a partial recipe
in el-get-sources didn't have a :description field.

fixes #1797

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Optimize el-get-read-all-recipe-names

This function is used to create completion candidates, so its slowness
is quite noticable. The main cause of slowness was the reading of all
the recipe files which can be avoided because the file name gives the
recipe name.