terraform-providers: fix handling of version/rev in provider list

I interpreted the purpose of stripping the first character from the 'version'
argument as an attempt to remove a prefixed 'v' (e.g. 'v1.0.0') from a version
tag. This works if the tag actually has a 'v' prefix, but also removes the first
character if version tags are not prefixed (e.g. '1.0.0').

Additionally, the 'v' was added again when specifying the `rev` for
`fetchFromGitHub` in default.nix. As described above, this did also not work
when provider repos did not prefix their version tags with 'v'.

I changed the implementation as follows:

- `version` and `rev` are stored inside data.nix
- `version` is used to declare the nix package version
- `rev` is used to fetch the proper git ref when building the package
- for determining `version`, an optional leading 'v' is trimmed from the tag
  name

Now this has the implication that the latest tag must always be a release tag
when using the `update-all` script, but as the result of running `update-all`
should always be reviewed before submission, makes this appear a manageable
tradeoff to me.