Prompts' fish_*.fish files removed during a full update when swapping prompts

[jorgebucaran]

Prompts' fish_*.fish files (fish_prompt, fish_right_prompt, fish_greeting, fish_title, fish_mode_prompt) get removed during fisher update if you try to replace the current prompt with another one.

Consider this scenario:

$ fisher list
jorgebucaran/fisher
ilancosman/tide

Then edit your $fish_plugins like so:

$EDITOR $fish_plugins

jorgebucaran/fisher
- ilancosman/tide
+ matgreen/lucid.fish

Run fisher update.

Fisher will install matgreen/lucid.fish first, overwriting ilancosman/tide's fish_*.fish files, then attempt to remove ilancosman/tide and we'll end up without a fish_prompt, etc.

We should be able to fix this by removing first, then installing/updating during a fisher update.