fix: exclude symlinks to directories from hashing

[10ko]

What this PR does / why we need it:
This PR fixes #1029, which was caused by bad handling of symlink when initializing garden.

Which issue(s) this PR fixes:

Fixes #1029

Special notes for your reviewer:
We use the git client to retrieve all the files from a project, from where to extract then the module config paths. The current implementation then doesn't allow the use of symlinks: the git client will never resolve a symlink if it's a directory. It will always treat it as file and (this is now fixed/prevented) would try to hash it using the node streams, failing.
From a design point of view, I am not 100% aware of all the reasons behinf choosing git ls-files instead of something else. What I find strange is the jump from "using a library" to retrieve files and "using bare node streams" to hash it. It would be nice to understand if it makes sense to convert the search function to the same node standard or viceversa if we can use the git client to do the hashing.

Does this PR introduce a user-facing change?:
No

[edvald]

Looks good, but it'd be great to have a simple test for this. I can imagine refactors accidentally breaking this at some point.

[10ko]

Makes sense, thanks!