lib: allow sourceByRegex to be composed after cleanSourceWith

`sourceByRegex src regexes` should include a source file if one of the
regular expressions `regexes` matches the path of that file relative
to `src`.

However to compute this relative path `sourceByRegex` uses:

```
relPath = lib.removePrefix (toString src + "/") (toString path);
```

Note that `toString path` evaluates to an absolute file somewhere
under `src` and not under `/nix/store`.

The problem is that this doesn't work if `src` is a `cleanSourceWith`
invocation as well because `toString src` will then evaluate to
`src.outPath` which will evaluate to `builtins.filterSource ...` which
evaluates to a path in `/nix/store` which is not a prefix of `path`.

The solution is to replace `src` with `origSrc` where

```
origSrc = if isFiltered then src.origSrc else src;
isFiltered = src ? _isLibCleanSourceWith;
```

Test this by executing the following from the nixpkgs repo:

```
(cat << 'EOI'
let
  pkgs = import ./. {};
in pkgs.runCommand "test-sourceByRegex" {
  test_sourceByRegex =
    let
      src1 = pkgs.lib.sourceByRegex ./.  [ "^test-sourceByRegex.nix$" ];
      src2 = pkgs.lib.sourceByRegex src1 [ "^test-sourceByRegex.nix$" ];
    in src2 + "/test-sourceByRegex.nix";
} ''
 cp $test_sourceByRegex $out
''
EOI
) > test-sourceByRegex.nix
nix-build test-sourceByRegex.nix
```