Possibly separate dependency roots from dependency definitions

[kriswuollett]

It seems like transitive and prune options are confusing in #100. I am also not sure that the use case of merely being an implementer of third party protos is supported as discovered in #138. So this issue is a possible feature request of specifying the dependency roots at a top level instead of in the repos.

To avoid any confusion I think it may be possible to just provide dependency roots from the combination of a list of fully qualified identifiers and a list of file paths for which each file contains its set of symbols.

The requirement for symbols is necessary if your system could support any number of messages that are opaquely wrapped inside Any protos...

Basically the config file could look like this:

name = "corp"
include_protos = ["grpc/health/v1/health.proto"]
include_symbols = ["envoy.service.ratelimit.v3.Authorization"]

[envoy]
url = "github.com/envoyproxy/envoy"
revision = "72d653e2540cc5f77e2acdc1c9a57a10263d74dc"
protocol = "https"
content_roots = ["api"]

[grpc]
url = "github.com/grpc/grpc"
# v1.64.0
revision = "b8a04acbbf18fd1c805e5d53d62ed9fa4721a4d1"
protocol = "https"
content_roots = ["src/proto"]

And trying to run fetch should fail until I also specify this for example, a dependency of envoy:

[udpa]
url = "github.com/cncf/udpa"
revision = "c52dc94e7fbe6449d8465faaeda22c76ca62d4ff"
protocol = "https"