fix: Improve workspace root detection. Fix config loading and validation.

[milesj]

Packemon is run in 4 different contexts:

• Polyrepo root (builds 1 package)
• Monorepo root (builds all packages)
• Monorepo root with filters (builds N packages)
• Monorepo project (builds 1 package)

The first 3 work just fine, as they were the intended developer flow for Packemon. However, the 4th is currently "broken". Packages build just fine, but configuration (package.config.js, etc) are resolved incorrectly, and any package.json validation (mainly repository) fails incorrectly.

To solve this problem, I must detect the "repo root".