`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
```
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