Support for multiple compose files

[jordanstephens]

Fixes #509 and #369

In #500, we were concerned that the presence of both a compose.yaml and docker-compose.yaml file could be confusing, so we decided to return an error and exit early. However, compose-go does log out a warning in this scenario, as does docker compose. We've changed our mind and decided to aim for closer parity with docker here by reverting to logging the warning and continuing with compose.yaml.

While implementing this, in an effort to bring us to closer parity with docker compose, we decided to go ahead and add support for passing -f multiple times—adding support for multiple compose files.

• Add support for multiple compose files with -f.
• Modify compose.LoadCompose to leverage the compose-go library to find default compose file paths if one isn't passed explicitly.
• Avoid returning an error if multiple default compose files are found. compose-go will always pick compose.yaml over docker-compose.yaml. It will also log a warning message explaining this to stderr.
• Remove toomany test case since there is nothing meaningful to test anymore.
• Add a simple test at the command level for something like defang version to make sure it doesn't try to load a compose file during intialization. In time, we should add command-level tests for everything with a mock server.
• lazily verify compose path, to avoid requiring it for commands like defang version which do not use it.