mirror nvim's package.path behavior for fennel resolution

[jaawerth]

In neovim, package.path is currently updated automatically based on nvim's runtimepath value.

• For parity with nvim's lua support, this behavior should be mirrored
• There should be a config option to disable this behavior if desired.

[jaawerth]

Did an initial pass on this in the dev branch and it seems to be working. Will close in master.

[jaawerth]

Done!