Update package imports to use internal package structure

[catatsuy]

This pull request primarily involves a reorganization of the codebase, moving several files and updating their references accordingly. The most significant changes are the relocation of the cli, config, and slack packages to an internal directory, and the subsequent updates to import statements and build configurations to reflect these changes.

Here are the key changes:

Codebase reorganization:

• .goreleaser.yml: Updated the ldflags configuration to reflect the new location of the cli package.
• Makefile: Updated the build rules for bin/notify_slack to include the new locations of the cli, config, slack, and throttle packages.
• cmd/notify_slack/main.go: Updated the import statement to reflect the new location of the cli package.

Updates to import statements:

• internal/cli/cli.go: Updated import statements to reflect the new locations of the config, slack, and throttle packages.
• internal/cli/cli_test.go: Updated import statements to reflect the new locations of the config and slack packages.
• internal/config/config_test.go: Updated import statement to reflect the new location of the config package.
• internal/slack/client_test.go: Updated import statement to reflect the new location of the slack package.