Introduce a deps and a setup macro (#481)

This allows us to pull in dependencies that we may need to ensure that
rules_jvm_external functions. At present, this is a no-op.

README.md

@@ -75,6 +75,14 @@ http_archive(
     url = "https://github.com/bazelbuild/rules_jvm_external/archive/%s.zip" % RULES_JVM_EXTERNAL_TAG,
 )
 
+load("@rules_jvm_external//:repositories.bzl", "rules_jvm_external_deps")
+
+rules_jvm_external_deps()
+
+load("@rules_jvm_external//:setup.bzl", "rules_jvm_external_setup")
+
+rules_jvm_external_setup()
+
 load("@rules_jvm_external//:defs.bzl", "maven_install")
 
 maven_install(
@@ -98,6 +106,14 @@ or environment variables. See the [Coursier
 documentation](https://get-coursier.io/docs/other-credentials.html#property-file)
 for more information.
 
+`rules_jvm_external_setup` uses a default list of maven repositories to download
+ `rules_jvm_external`'s own dependencies from. Should you wish to  change this, 
+ use the `repositories` parameter:
+
+ ```python 
+rules_jvm_external_setup(repositories = ["https://mycorp.com/artifacts"])
+```
+
 Next, reference the artifacts in the BUILD file with their versionless label:
 
 ```python

WORKSPACE

@@ -51,8 +51,20 @@ http_archive(
 # dependencies. So, we omit them to keep the WORKSPACE file simpler.
 # https://skydoc.bazel.build/docs/getting_started_stardoc.html
 
+load("//:repositories.bzl", "rules_jvm_external_deps")
+
+rules_jvm_external_deps()
+
+load("//:setup.bzl", "rules_jvm_external_setup")
+
+rules_jvm_external_setup()
+
+# Begin test dependencies
+
 load("//:defs.bzl", "maven_install")
 
+load("//:specs.bzl", "maven")
+
 maven_install(
     name = "outdated",
     artifacts = [
@@ -63,10 +75,6 @@ maven_install(
     ],
 )
 
-# Begin test dependencies
-
-load("//:specs.bzl", "maven")
-
 maven_install(
     artifacts = [
         "com.google.guava:guava:27.0-jre",

repositories.bzl

@@ -0,0 +1,16 @@
+load("//:defs.bzl", "maven_install")
+
+_DEFAULT_REPOSITORIES = [
+    "https://repo1.maven.org/maven2",
+    "https://jcenter.bintray.com/",
+    "https://maven.google.com",
+]
+
+def rules_jvm_external_deps(repositories = _DEFAULT_REPOSITORIES):
+    maven_install(
+        name = "rules_jvm_external_deps",
+        artifacts = [
+        ],
+        maven_install_json = "@rules_jvm_external//:rules_jvm_external_deps_install.json",
+        repositories = repositories,
+    )

rules_jvm_external_deps_install.json

@@ -0,0 +1,9 @@
+{
+    "dependency_tree": {
+        "__AUTOGENERATED_FILE_DO_NOT_MODIFY_THIS_FILE_MANUALLY": 2914,
+        "conflict_resolution": {},
+        "dependencies": [
+        ],
+        "version": "0.1.0"
+    }
+}

setup.bzl

@@ -0,0 +1,4 @@
+load("@rules_jvm_external_deps//:defs.bzl", "pinned_maven_install")
+
+def rules_jvm_external_setup():
+    pinned_maven_install()