How to do it

1. Create a root level directory called buildSrc.
2. Inside buildSrc create a file called build.gradle.kts and enable the kotlin-dsl plugin with the snippet below. Don’t forget to do a gradle sync to activate this plugin.

import org.gradle.kotlin.dsl.`kotlin-dsl`
plugins {
    `kotlin-dsl`
}

1. Create directories src > main > java inside buildSrc .
2. Inside java, create a Kotlin file and give it any name you want (Dependencies.kt for example).
3. Create as many object declarations as you want (ideally at least 1 for Versions and another for Dependencies).

Example

object Versions {
    val kotlin = "1.2.21"
    val support_lib = "27.0.2"
    val retrofit = "2.3.0"
    val rxjava = "2.1.9"
}

object Dependencies {
    val kotlin_stdlib = "org.jetbrains.kotlin:kotlin-stdlib:${Versions.kotlin}"
    val support_annotations = "com.android.support:support-annotations:${Versions.support_lib}"
    val support_appcompat_v7 = "com.android.support:appcompat-v7:${Versions.support_lib}"
    val retrofit = "com.squareup.retrofit2:retrofit:${Versions.retrofit}"
    val retrofit_rxjava_adapter = "com.squareup.retrofit2:adapter-rxjava2:${Versions.retrofit}"
    val rxjava = "io.reactivex.rxjava2:rxjava:${Versions.rxjava}"
}

You will be able to add this dependency straight away (with the help of autocomplete & navigation) across all of your modules with:

implementation Dependencies.kotlin_stdlib
implementation Dependencies.support_annotations
implementation Dependencies.support_appcompat_v7
implementation Dependencies.retrofit
implementation Dependencies.retrofit_rxjava_adapter
implementation Dependencies.rxjava

Credits: https://caster.io/lessons/gradle-dependency-management-using-kotlin-and-buildsrc-for-buildgradle-autocomplete-in-android-studio.