This fix a problem with the garbage collector not being able to
clean the MetaElements that are loaded.

On small projects this is not a problem, but in bigger projects, this
can save a few hundred of MBs at runtime

The reason behind this is, whenever we have a "stack" element, which
has no stack.yaml configuration, since it doesn't need it, we would
get an exception thrown when initiating the first one, as loading the
yaml file would fail.

This would capture the frame in which this command was executed, which
references meta_elements. Therefore, as long as another exception is not
thrown, the garbage collector would not be able to clean all the
MetaElements.

Cleanup MetaElement local state

See merge request !1147

Resolving symlinks during staging causes various issues:

* Split rules may not work properly as the resolved paths will differ
  depending on whether another artifact with a directory symlink has
  been staged in the same root directory or not, e.g., as part of
  compose.

* The order of symlinks in file lists is difficult to get right to
  guarantee consistent and predictable behavior as paths in a file list
  might rely on symlinks in the same file list. See #647 and #817.

* Staging order differences can lead to surprising results. See #390.

* Difficult to properly support absolute symlinks. Absolute symlinks are
  currently converted to relative symlinks, however, this doesn't always
  work. See #606 and #830.

This will require changes in projects that rely on the current behavior.
However, the changes are expected to be small and are often a sign of
buggy element files. E.g., elements that don't fully obey `bindir` or
`sbindir` variables.

This matches the change in utils._process_list().

This also removes the _Resolver class as it is now unused. We may want
to support controlled symlink resolution in the future, in which case
the _Resolver class can be resurrected from this commit.

Copy symlinks as they are, absolute or relative. We no longer resolve
symlinks when copying files, which makes this safe.