Allow git repos to be submodules

[schmir]

I'm using yadm and would like to track the plugins as submodules of the yadm repository. When checking out such a submodule, .git is not a directory, but rather a file. This resulted in the following error, for each of the plugins that I track as a submodule:

% exec zsh
.znap.source:16: no directory expansion: ~[superbrothers/zsh-kubectl-prompt] 
.znap.source:16: no directory expansion: ~[mattmc3/zfunctions] 
.znap.source:16: no directory expansion: ~[olets/zsh-abbr] 
.znap.source:16: no directory expansion: ~[Aloxaf/fzf-tab]

With this commit ..znap.repos and ..znap.dirname handle that case. Please note that there are other checks in some of the scripts that do also check for a .git directory. They may still need to be changed.

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• Each commit message includes Fixes #<bug> or Resolves #<issue> in its body (not subject, that is, the
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