Stop removing all hidden files

[marcoceppi]

currently, charm strips all hidden files from a charm payload but this is removing valuable information that's not related to VCS.

This strips files, like .build.manifest, which are used to record the layers in the built charm artifact.

[ryan-beisner]

OpenStack Charms CI also needs the .build.manifest to survive charm push so that the published artifact possesses the manifest (and other .files).

I believe the relevant code is @ https://github.com/juju/charm/blob/v5/charmdir.go