make harpoon-go-to work when in harpoon-file

[emillenz]

harpoon-go-to should work as expected inside harpoon-file

i think it would be good default behaviour to make harpoon-go-to also work as expected (to jump to the file associated with the number) when inside the harpoon-file buffer of the project.

since most users have bound harpoon-go-to-1 etc. to a fixed keybind, which doesn't not work when inside the harpoon-file.

current erroneous behaviour

currently harpoon errors when trying this, since the file HARPOON-CACHE-FILE/harpoon doesn't exist. this happens because harpoon reads the HARPOON-CACHE-FILE itself as a project and thus tries to open its harpoon-file, which obviously doesn't exist.
(there was also a bug where (eq major-mode 'harpoon-mode) is called inside (with-temp-buffer) and thus is always nil.)

test it

follow these steps, one time with, and one time without the proposed changes:
1. navigate to an existing project
2. call harpoon-toggle-file
3. call eg. harpoon-go-to-1 and observe behaviour