Add Helm source for projectile-current-project-dirs, change helm-projectile-sources-list priority and allow helm-projectile to use outside of a project

Helm source for listing project directories similar to `projectile-find-dir`, but with more actions. Default is Dired.

`helm-source-projectile-buffers-list` and `helm-source-projectile-recentf-list` should be prioritized higher, since those two have smaller candidate space. Otherwise, those two will never be used because helm-source-projectile-files-list is a superset of both, and candidates in helm-source-projectile-files-list are always displayed first, making the two smaller sources almost never be used, unless explicitly move to that source by arrow keys which is less convenient.

`helm-projectile` when used outside of a project simply displays a list of known projects.