be more precise about elpa package dir names

ELPA Package directories are named `<package>-<version>`.  Previously
`el-get-elpa-package-directory` looked for package-dirs using
`all-completions` meaning anything starting with `<package>` would match.
This is problematic if a package is a prefix of another package, eg the
`s` and `solarized-theme` packages.

Instead of doing that, look for a directory that is precisely
`<package>-<version>` where `<version>` is something acceptable to
`version-to-list`

Fixes #1189.