Add more tests for Copy and some fixes.

[tejal29]

Fixes to #1111 , #1110
In #1111, @gilbsgilbs metioned, copy symlink files should not be evaluated.

I started testing what docker build does and found

1. docker build creates symlinks when copying a single file which is a symlink
   see Dockerfile COPY from image resolves symbolic links moby/moby#40449
2. docker build preserves the symlink when copying a dir.

Kaniko, copy command was always creating a regular file.

While adding tests for copyCommand i found few issues.

1. If dest was a directory in the context workspace, it wasn't identified as dir.
2. Missed errors.Wrap that i found while debuging issues.

Please see all the scenarios tested with docker build here #1111 (comment)