Improve file navigation in a project

[ThomasFeher]

Two fundamental approaches possible:

1. Extend a filebrowser like NerdTree to only show valid directories for the
   current project.
2. Extend search plug-ins like fzf-vim to only search in valid directories for
   the current project.

Approach 1 seems more effort and as I currently do not use a filebrowser at all
it seems not worth it.

Approach 2 seems to be possible with little effort, as it is a matter of
configuration. fzf, unfortunately, does not take multiple directories for the
search (it changes the CWD instead, see pushd called here),
although ag and ripgrep seem to allow that.

ripgrep provides option glob which must be satisfied for any found file to
become a valid result.
With this we could always search in dev/ and add only
directories relevant for the current project to the glob option.

ag doesn't seem to provide any such option.