Show enum defaults in `--help` output (#693)
With `Option<T>` and `.unwrap_or_default()` later, the default of `T`
isn't shown in the help output.
Old:
```
--link-mode <LINK_MODE>
The method to use when installing packages from the global cache
Possible values:
- clone: Clone (i.e., copy-on-write) packages from the wheel into the site packages
- copy: Copy packages from the wheel into the site packages
- hardlink: Hard link packages from the wheel into the site packages
-q, --quiet
Do not print any output
--resolution <RESOLUTION>
Possible values:
- highest: Resolve the highest compatible version of each package
- lowest: Resolve the lowest compatible version of each package
- lowest-direct: Resolve the lowest compatible version of any direct dependencies, and the highest compatible version of any transitive dependencies
--prerelease <PRERELEASE>
Possible values:
- disallow: Disallow all pre-release versions
- allow: Allow all pre-release versions
- if-necessary: Allow pre-release versions if all versions of a package are pre-release
- explicit: Allow pre-release versions for first-party packages with explicit pre-release markers in their version requirements
- if-necessary-or-explicit: Allow pre-release versions if all versions of a package are pre-release, or if the package has an explicit pre-release marker in its version requirements
```
New:
```
--link-mode <LINK_MODE>
The method to use when installing packages from the global cache
[default: hardlink]
Possible values:
- clone: Clone (i.e., copy-on-write) packages from the wheel into the site packages
- copy: Copy packages from the wheel into the site packages
- hardlink: Hard link packages from the wheel into the site packages
-q, --quiet
Do not print any output
--resolution <RESOLUTION>
[default: highest]
Possible values:
- highest: Resolve the highest compatible version of each package
- lowest: Resolve the lowest compatible version of each package
- lowest-direct: Resolve the lowest compatible version of any direct dependencies, and the highest compatible version of any transitive dependencies
--prerelease <PRERELEASE>
[default: if-necessary-or-explicit]
Possible values:
- disallow: Disallow all pre-release versions
- allow: Allow all pre-release versions
- if-necessary: Allow pre-release versions if all versions of a package are pre-release
- explicit: Allow pre-release versions for first-party packages with explicit pre-release markers in their version requirements
- if-necessary-or-explicit: Allow pre-release versions if all versions of a package are pre-release, or if the package has an explicit pre-release marker in its version requirements
```
