In order to remove circular references between subspecs of libwebp, had to restructure the subspecs (created core). For details, see https://github.com/CocoaPods/Specs/issues/12819
For backwards compatibility, applied the same fix for the libwebp specs 0.4.1 and 0.4.0. Didn't change 0.3.1 and 0.3.0-rc7 because those 2 no longer compile with arm64. Will need to find a way to remove/deprecate them.
Reverted changes for 0.4.0 as well, this one doesn't compile for arm64 either.
Merge pull request #12820 from bpoplauschi/libwebp_remove_circular_refs

libwebp - remove circular references between subspecs