implement CLI option to map proto imports to their module in the project.

[james-lawrence]

currently imports of other proto files assumes the same relative path. if that assumption doesn't hold there doesn't seem to be a way to remap the path for the dependency.

see related: stephenh/ts-proto#596

[yibo-long]

+1, it will be great for the support.

We manage our protos with different languages with dependencies. For example there is proto NPM package @spanio/a and proto NPM package @spanio/b with b depending on a as:

syntax = "proto3";
package io.span.b;

import "io/span/a/foo.proto";

message Bar {
  io.span.a.Foo foo = 1;
}

some option support like Mio/span/a/foo.proto=@spanio/a will be great! Maybe not prefered by everyone, but Mio/span/a=@spanio/a could make our lives much easier.