Improves some of the "no versions of <package> are available" messages
by showing the complement or inversion of the package.
Does not address cases like
```
Because there are no versions of crow that satisfy any of:
crow>1.0.0,<2.0.0a5
crow>2.0.0a7,<2.0.0b1
crow>2.0.0b1,<2.0.0b5
...
```
which are a bit more complicated; I'll focus on those cases in a
follow-up.
## Summary
I don't know if this is actually a good change, but it tries to make the
editable install experience more consistent. Specifically, we now
support...
```
# Use a relative path with a `file://` prefix.
# Prior to this PR, we supported `file:../foo`, but not `file://../foo`, which felt inconsistent.
-e file://../foo
# Use environment variables with paths, not just URLs.
# Prior to this PR, we supported `file://${PROJECT_ROOT}/../foo`, but not the below.
-e ${PROJECT_ROOT}/../foo
```
Importantly, `-e file://../foo` is actually not supported by pip... `-e
file:../foo` _is_ supported though. We support both, as of this PR. Open
to feedback.
Zanie
Charlie Marsh