Flatten requirements eagerly in `get_dependencies` (#4430)
Downstack PR: #4515 Upstack PR: #4481
Consider these two cases:
A:
```
werkzeug==2.0.0
werkzeug @ https://files.pythonhosted.org/packages/ff/1d/960bb4017c68674a1cb099534840f18d3def3ce44aed12b5ed8b78e0153e/Werkzeug-2.0.0-py3-none-any.whl
```
B:
```toml
dependencies = [
"iniconfig == 1.1.1 ; python_version < '3.12'",
"iniconfig @ git+https://github.com/pytest-dev/iniconfig@93f5930e668c0d1ddf4597e38dd0dea4e2665e7a ; python_version >= '3.12'",
]
```
In the first case, `werkzeug==2.0.0` should be overridden by the url. In
the second case `iniconfig == 1.1.1` is in a different fork and must
remain a registry distribution.
That means the conversion from `Requirement` to `PubGrubPackage` is
dependent on the other requirements of the package. We can either look
into the other packages immediately, or we can move the forking before
the conversion to `PubGrubDependencies` instead of after. Either version
requires a flat list of `Requirement`s to use. This refactoring gives us
this list.
I'll add support for both of the above cases in the forking urls branch
before merging this PR. I also have to move constraints over to this.
