feature request: $version in source-repository/tag

I have my version number in two places in many of my cabal files:

```
version: a.b.c
source-repository this
tag: a.b.c
```

It is annoying and somewhat error-prone to have this duplication: one must remember to update both places when making a release. It would be nice if there were a variable or something that could be inserted here to reduce the duplication; e.g. something like

```
version: a.b.c
source-repository this
tag: $version
```

would be much nicer and somewhat consistent with cabal-install's configuration file format.

The issue names Cabal files and the version and source-repository tag fields, but no repository files or tests. Start by locating the parsing and validation paths for these fields, then determine the intended expansion and error behavior for a version reference. Done means the duplication can be removed while existing literal tags remain valid and the behavior is covered by tests.

Written by the indexing model from the issue text.