This patch adds `lib.repoRevToName` function that generalizes away most of the
code used for derivation name generation by `fetch*` functions (`fetchzip`,
`fetchFromGitHub`, etc, except those which are delayed until latter commits
for mass-rebuild reasons).
It's first argument controls how the resulting name will look (see below).
Since `lib` has no equivalent of Nixpkgs' `config`, this patch adds
`config.fetchedSourceNameDefault` option to Nixpkgs and then re-exposes
`lib.repoRevToName config.fetchedSourceNameDefault` expression as
`pkgs.repoRevToNameMaybe` which is then used in `fetch*` derivations.
The result is that different values of `config.fetchedSourceNameDefault` now
control how the `src` derivations produced by `fetch*` functions are to be
named, e.g.:
- `fetchedSourceNameDefault = "source"` (the default):
```
$ nix-instantiate -A fuse.src
/nix/store/<hash>-source.drv
```
- `fetchedSourceNameDefault = "versioned"`:
```
$ nix-instantiate -A fuse.src
/nix/store/<hash>-libfuse-2.9.9-source.drv
```
- `fetchedSourceNameDefault = "full"`:
```
$ nix-instantiate -A fuse.src
/nix/store/<hash>-libfuse-2.9.9-github-source.drv
```
See the documentation of `config.fetchedSourceNameDefault` for more info.
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