[FR]: Add option to copy_to_directory to flatten the output tree structure

[aaronsky]

What is the current behavior?

Directories input to this rule are copied while retaining the previous tree structure, which is opaque to Bazel.

Describe the feature

It would be useful for me to add a flatten field to the rule, as input to the copy_to_directory binary. This option would produce the flattened representation of the input directory tree as output, resulting in a set of files with no internal nesting.

[kormide]

@aaronsky You can accomplish a flatten operation by doing:

copy_to_directory(
    name = "flattened",
    srcs = ["some_dir"],
    root_paths = ["%s/some_dir/**" % package_name()],
)

I'll close the issue assuming this addresses your use case, but if not then feel free to re-open.