This change fixes a couple bugs with assets:
* We weren't recursing into subdirectories in the new "path as
archive" feature, which meant we missed most of the files.
* We need to make paths relative to the root of the archive
directory itself, otherwise paths end up redundantly including
the asset's root folder path.
* We need to clean the file paths before adding them to the
archive asset map, otherwise they are inconsistent between the
path, tar, tgz, and zip cases.
* Ignore directories when traversing zips, since they aren't
included in the other formats.
* Tolerate io.EOF errors when reading the ZIP contents into blobs.
* Add test cases for the four different archive kinds.
This fixespulumi/pulumi-aws#50.
