libwebp - remove circular references between subspecs

In order to remove circular references between subspecs of `libwebp`, had to restructure the subspecs (created `core` subspec with all the files from `dsp`, `utils`, `enc` and `dec`. Kept those for backwards compatibility). For details, see https://github.com/CocoaPods/Specs/issues/12819.

@kylef There are some older pod specs in `libwebp`: `0.3.0-rc7`, `0.3.1` and `0.4.0` that don't compile for arm64 which breaks the Travis CI. Since those tags from libwebp are not to be used by CocoaPods users, we should delete them or mark them as deprecated in some way. How can we do that? I thought about deleting them, but I'm not sure about the impact on users that have them. Could we use the `s.deprecated` setting? Would it work if we'd change the older specs to have no source files and `s.deprecated = true`, so they compile fine and show the user the version is deprecated?

Note: since I am only the person to push specs to CocoaPods and not the maintainer of this library, I cannot make changes inside their code, so I'm stuck with updating the specs.