Add support for the =~ constraint

[egibs]

We were trying to use the =~ constraint via the Apko Terraform Provider which should be valid:

/ # apk add yara-x=~0.14.0
(1/1) Installing yara-x (0.14.0-r3)
Executing busybox-1.37.0-r40.trigger
OK: 166 MiB in 16 packages

but this was not a valid matcher case so we defaulted to versionAny which would instead pull in the latest version.

This PR adds a case for the =~ constraint and treats it like ~ so that we correctly resolve versions via includesVersion and updates a few tests to validate both the ~ and =~ constraints.

FWIW it does look like ~ works as expected.