lazily call rules_python_external_dependencies

This avoids users having to add a new WORKSPACE dependency call in order to use pip_install

Author: Alex Eagle

WORKSPACE

@@ -14,10 +14,6 @@
 
 workspace(name = "rules_python")
 
-load("//python:pip.bzl", "pip_repositories")
-
-pip_repositories()
-
 # Everything below this line is used only for developing rules_python. Users
 # should not copy it to their WORKSPACE.
 
@@ -28,8 +24,3 @@ rules_python_internal_deps()
 load("//:internal_setup.bzl", "rules_python_internal_setup")
 
 rules_python_internal_setup()
-
-# TODO(alexeagle): vendor these dependencies so we don't expose new ones to users
-load("//experimental/rules_python_external:repositories.bzl", "rules_python_external_dependencies")
-
-rules_python_external_dependencies()

examples/rules_python_external/WORKSPACE

@@ -12,10 +12,6 @@ load("@rules_python//python:repositories.bzl", "py_repositories")
 
 py_repositories()
 
-load("@rules_python//experimental/rules_python_external:repositories.bzl", "rules_python_external_dependencies")
-
-rules_python_external_dependencies()
-
 load("@rules_python//experimental/rules_python_external:defs.bzl", "pip_install")
 
 pip_install(

experimental/rules_python_external/defs.bzl

@@ -1,6 +1,6 @@
 ""
 
-load("//experimental/rules_python_external:repositories.bzl", "all_requirements")
+load("//experimental/rules_python_external:repositories.bzl", "all_requirements", "rules_python_external_dependencies")
 
 DEFAULT_REPOSITORY_NAME = "pip"
 
@@ -179,6 +179,9 @@ def pip_install(requirements, name = DEFAULT_REPOSITORY_NAME, **kwargs):
       name: A unique name for the created external repository (default 'pip').
       **kwargs: Keyword arguments passed directly to the `pip_repository` repository rule.
     """
+    # Just in case our dependencies weren't already fetched
+    rules_python_external_dependencies()
+
     pip_repository(
         name = name,
         requirements = requirements,