Fixes#9001#8289 added support for install_tree and copy_tree to merge into an existing
directory structure. However, it did not properly handle relative symlinks and
also removed support for the 'ignore' keyword. Additionally, some of the tests
were overly-strict when checking the permissions on the copied files.
This updates the install_tree/copy_tree methods and their tests:
* copy_tree/install_tree now preserve relative link targets (if the symlink in the
source directory structure is relative, the symlink created in the destination
will be relative)
* Added support for 'ignore' argument back to copy_tree/install_tree (removed
in #8289). It is no longer the object output by shutil.ignore_patterns: you pass a
function that accepts a path relative to the source and returns whether that
path should be copied.
* The openfoam packages (currently the only ones making use of the 'ignore'
argument) are updated for the new API
* When a symlink target is absolute, copy_tree and install_tree now rewrite the
source prefix to be the destination prefix
* copy_tree tests no longer check permissions: copy_tree doesn't enforce
anything about permissions so its tests don't check for that
* install_tree tests no longer check for exact permission matching since it can add
file permissions
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