There's three phases of code-import-ability in a Mach invocation:

1. Pre-initialization: We need to parse the list of module locations from mach_virtualenv_packages.txt and manage the virtualenv without having access to the other modules yet. (note: there's code in here that may be used in #2 as well, it's not only for initialization).
2. Post-initialization: Most Mach code runs here and has access to global dependencies and all needed first-party modules.
3. Command: Command-specific dependencies are available

Right now, #1 and #2 are smudged together, and it's easy to accidentally break the initialization process by adding an import to code that only executes in #1.