lib, treewide: introduce `repoRevToName`, use it in most `fetch*` functions

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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