fix: panic in offline linter + handling stdin

[julienduchesne]

Follow-up to #10059 and #10559

The offline linter is working well in ideal scenarios, I am linting a set of files/directory in 5 seconds where it used to take ~5 minutes

However, I did find issues after testing some more:
Given an non-existent file, it panics (I inverted the info.IsDir() statement and the one checking for errors)
There's a disconnect between the filepath.Walk in the linter and the one building the offline client which means that there were other issues (such as stdin not working)

To fix those issues once and for all, I extracted the walk func to a common path (file.WalkManifests); that way, we are sure that changes are always reflected in both locations

I also added some tests for non-existent files as well as offline linting from stdin

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[tooptoop4]

@julienduchesne is there an argo binary anywhere with this fix? or do I need to compile the binary?