Refactor compatibility

[LinqLover]

This PR reworks the structure of compatibility packages for Squot. The new semantics are:

• SquotCompatibility-SqueakCommon: contains compatibility extensions for all Squeak versions. Will only be loaded in Squeak. Contains most of the stuff which was previously in pharo-compatibility (CRC/Checksum, extensions on ByteArray and Collection).
• SquotCompatibility-Squeak50 and SquotCompatibility-Squeak60 contain version-specific compatibility extensions for Squeak and will be loaded only if needed. They both currently contain two extensions on MCRepositoryGroup copied from Monticello-ct.755 (inbox) which are required for #328. Once Monticello-ct.755 is merged into the Trunk, we can remove these extensions from the SquotCompatibility-Squeak60 package.
• SquotCompatibility-Pharo: currently empty, will only be loaded in Pharo. Placeholder for future compatibility extensions for Pharo.

The package pharo-compatibility is removed.

Please squash when merging.

[LinqLover]

@j4yk Would you mind reviewing this again? PR description is updated, please squash when merging :)

[j4yk]

Merged with amendments in 21e866477a037df3577c5737bc4fae2e7f9a2908

[LinqLover]

Yay, finally green CIs again!