buf config init that can parse the Protobuf roots of a given source directory

[doriable]

Feature

Having buf config init take a directory (. by default), finds all Protobuf definitions, and create a buf.yaml with the appropriate modules.

For example, I have a source repository for my project that might look something like:

.
├── LICENSE
├── README.md
├── proto
│   ├── api
│   └── vendor
└── src

Running buf config init, it would:

• Find all the Protobuf definitions
• Scans the import paths of all the files
• Figures out that there are two Protobuf roots, api and vendor
• Produce a buf.yaml with the following content:

version: v2
modules:
  - path: proto/api
  - path: proto/vendor

This is pretty closely related to #3159