Handle :builtin correctly in `el-get-describe'.

So far :builtin is ignored by `el-get-describe' and the regular
install method is always displayed.  With this patch the builtin
information is instead displayed.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Improve `el-get-describe' of :builtin packages.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #1219 from ruediger/better-builtin-describe

Handle :builtin correctly in `el-get-describe'.