Change module path to classic-terra/core

[inon-man]

Summary of changes

Original PR is #43, Note: protobuf changes were not included.

This change is needed for me to do "go get github.com/classic-terra/classic" in my db-benchmark. If kept "github.com/terra-money/core", go would fetch Terra 2.0

Report of required housekeeping

• Github issue OR spec proposal link
• Wrote tests
• Updated API documentation (client/lcd/swagger-ui/swagger.yaml)
• Added a relevant changelog entry: clog add [section] [stanza] [message]

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(FOR ADMIN) Before merging

• Added appropriate labels to PR
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Confirm added tests are consistent with the intended behavior of changes
• Ensure all tests pass

[inon-man]

Oh sorry that I accidentally pushed merge button. If you guys find any issue, I will make a revert PR