Unify editable and unnamed URL parsing (#3946)
## Summary
This will help prevent bugs like #3934 by unifying the implementations
for editables and non-editable unnamed requirements. Specifically, both
of these now go through the same parsing paths and use the same struct
representations (with the exception that the editable flag is flipped in
the first case):
```
-e ./foo/bar
./foo/bar
```
We also now support PEP 508 in editable URLs. It turns out this is just
a limitation in pip, so it's correct to support it. For example, this
now works:
```
-e black[d] @ file://${PROJECT_ROOT}/scripts/packages/black_editable
```
Closes #3941.
Closes #3942.
