Fix el-get-elpa-package-directory returning a non-existing directory.

el-get-elpa-package-directory will not always return a real directory,
since two directories might have the same prefix (for example, erc
installs as `erc-5.3` but has a package name of `erc`. So
`try-completion` will return `erc-` if you have something
like `erc-image` installed.)

`all-completions` will return a list of actual directories which
satisfy the completion, and the `car` of this list will be the package
we want. `when realnames` will return `nil` when there are no possible
completions.