Auto merge of #4229 - azerupi:install-version-range, r=matklad

Implement semver ranges for install --vers

This implements the design discussed in #4207
It allows to specify semver ranges on `cargo install ... --vers`

1. The first character of the `--vers` value is checked, if there is none we return an error.
2. If it is one of `<`, `>`, `=`, `^`, `~` we parse the value as a `VersionReq`, otherwise we parse it as a `Version`.
3. If the parsing as a `Version` fails but parsing as `VersionReq` succeeds, we add a note to the warning mentioning that a qualifier should be used to specify a semver range. This catches versions with less than tree digits.

Otherwise, the previous behaviour is preserved with the warning of backwards compatibility.

This means that

- `cargo install ... --vers "^1.2.3"` will be parsed as a range
- `cargo install ... --vers 1.2.3` will be parsed as a version
- `cargo install ... --vers 1.2` will be parsed as a version for backwards compatibility reasons, fail and be passed through as is,**but** we will add a note `if you want to specify semver range, add an explicit qualifier, like ^1.2`
- `cargo install ... --vers blah` will be parsed as a version for backwards compatibility reasons (which is weird because it is not even a valid semver range) and produce an `unknown error` down the line. I have left this behaviour untouched because it worked like that before, but I can easily make it error sooner with a better message.