`pip.parse` should allow hiding transitive dependencies

# 🚀 feature request

### Relevant Rules

`pip.parse`

### Description

Given a `requirements.in` file containing only `foo==4.2.0` from which I create a lock file with content
```
foo==4.2.0 --hash=
dep_of_foo==1.33.7 --hash=
```

When using `pip.parse` to crate a hub `@pypi` from this lock file users can access all Python modules from the lock file. Concretely, `@pypi//foo` and `@pypi//dep_of_foo`.

I consider `dep_of_foo` an implementation detail which no user should depend on. When changing the version of `foo`, then `dep_of_foo` might vanish or change drastically as side effect. If I wanted users to access `dep_of_foo`, I would have added it to the `requirements.in` file to make it an explicit and desired direct dependency of my project.

It would be great if there were an option enforcing that transitive dependencies are not available to users.

### Describe the solution you'd like

Ideally `pip.parse` would offer an attribute `restrict_visibility_to` (or any other name) which takes a file list. Then, one could provide one or multiple `requirements.in` files to this attribute. `pip.parse` can then read those files, extract the Python module names and ensure only those are public targets in the pip hub.

The implementation would be easier if `restrict_visibility_to` takes a list of strings and the user explicitly states which Python modules should be public. However, I consider this inferior, as it increases the maintenance burden whenever changing the `requirements.in` files.

### Describe alternatives you've considered

I implemented the described behavior locally as a workspace rule, which creates a new hub with alias targets pointing to the hub created by `pip.parse`. It is trivial to do so, not much logic is required.

However, this means one has to teach people not to use the original hub created by `pip.parse`. Or one has to write yet another piece of custom code for a BUILD file checker ensuring this rule.
Overall, it would be much nicer if this behavior would be a feature of upstream `pip.parse`.

Start at the `pip.parse` rule and compare the `requirements.in` inputs with the generated lock file and pip hub targets. Review the local workspace rule and BUILD-file checker alternative described in the issue to understand the intended visibility boundary. Done means direct dependencies remain public while transitive dependencies are hidden without requiring duplicate custom rules.

Scritto dal modello di indicizzazione a partire dal testo della issue.