consolidate omicron-dev, omdb, and xtask into "dev-tools" directory

[davepacheco]

In the spirit of #3350. Prior to this, we had:

• omdb package at the top level for the omdb program
• dev-tools package at the top level that only contained the omicron-dev program.
• xtask package at the top level that only contains the xtask program.

The intent of dev-tools was to be a package that we could put other binaries in besides omicron-dev. But when I started putting omdb into it, I realized it needed a pretty different set of dependencies and that there was no reason to put them in the same package. So after this change:

• "dev-tools" is a directory at the top level, not a Rust package
• the omdb, omicron-dev, and xtask Rust packages are now inside "dev-tools"