a.proto

syntax = "proto3";

import "b.proto";

package api;

message Foo {
  Bar bar = 1;
}

b.proto

syntax = "proto3";

package api;

message Bar {}

main.go

package main

import (
	"log"

	"github.com/jhump/protoreflect/desc/protoparse"
)

func main() {
	p := protoparse.Parser{ImportPaths: []string{".", "api"}}
	_, err := p.ParseFiles("api/a.proto", "api/b.proto")
	if err != nil {
		log.Fatal(err)
	}
}