`poetry build` follows symlinks for files but not for directories

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: MacOS Catalina, gitlab/gitlab-runner:latest
• Poetry version: 1.0.3
• Link of a Gist with the contents of your pyproject.toml file:

Issue

We are changing package structure of our project and want to keep some compatibility aliases via symlinks to make migration easier. Project structure is the following (package_c contains symlinks only):

package_a:
    module_aa
    module_ab
package_b:
    module_ba
    module_bb
package_c:
    package_a  # --> ../package_a
    module_ba  # --> ../package_b/module_ba
    module_bb  # --> ../package_b/module_ba

pyproject.toml contains following lines:

packages = [
    { include = "package_a", from = "src" },
    { include = "package_b", from = "src" },
    { include = "package_c", from = "src" }
]

Resulting wheel contains copies of module_ba and module_bb at package_c path (which is fine) but package_c/package_a path is ignored completely.

Current behavior: file symlinks are silently followed, targets are copied; directory symlinks are silently ignored

Expected behavior:
One of these:

1. All symlinks inside project directory are followed and targets are copied to wheel
2. Symlinks are ignored, warning printed during build
3. Symlinks are prohibited, attempt to build project containing symlinks fails

I know symlinks in wheels have no official support (https://github.com/pypa/pip/issues/3500) but it would be nice if poetry will be at least more verbose when handling them.

Start by reproducing the issue with the shown package_a, package_b, and package_c symlink structure and the pyproject.toml packages configuration, then run poetry build and inspect the generated wheel. Trace how file and directory symlinks are handled during the build; done means the behavior is made consistent and matches one of the stated handling policies.