Enables cross-referencing of files in WriteFileStep

[ikskuh]

Also fixes bug in add InstallFileWithDir.

This PR allows one to use a WriteFileStep to write several related files:

write_step.addCopyFile("data/embed_1.bin", …);
write_step.addCopyFile("data/embed_2.bin", …);
write_step.add("bundle.zig",
    \\pub const embed1 = @embedFile("data/embed_1.bin");
    \\pub const embed2 = @embedFile("data/embed_2.bin");
);