feat(pip_repository): Enable PyPi dep cycles

This patch adjusts the pip_repository interface to accept a new
parameter: `composite_libs`, being a list of PyPi package names which
form a cycle and must be installed together.

The intuition behind this design is that a dependency cycle {a <-> b}
is implemented simply as emplacing both a and b at once. Hence a
dependency graph {c -> a, c -> b} has the same effect.

If we modify the installation of a and b to remove their mutual
dependency, and generate a c which dominates a and b, we can then modify
the `requirement()` and `whl_requirement()` helper functions to
recognize the requirements a and b and provide a reference to c instead.

examples/pip_parse_vendored/BUILD.bazel

@@ -1,6 +1,8 @@
 load("@bazel_skylib//rules:diff_test.bzl", "diff_test")
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
+load("@rules_python//python:defs.bzl", "py_test")
 load("@rules_python//python:pip.bzl", "compile_pip_requirements")
+load("//:requirements.bzl", "requirement")
 
 # This rule adds a convenient way to update the requirements.txt
 # lockfile based on the requirements.in.
@@ -15,13 +17,15 @@ genrule(
     outs = ["requirements.clean.bzl"],
     cmd = " | ".join([
         "cat $<",
-        # Insert our load statement after the existing one so we don't produce a file with buildifier warnings
-        """sed -e '/^load.*/i\\'$$'\\n''load("@python39//:defs.bzl", "interpreter")'""",
+        # Insert our load statement for the interpreter
+        """sed -e '7 i \\\nload("@python39//:defs.bzl", "interpreter")\n'""",
         # Replace the bazel 6.0.0 specific comment with something that bazel 5.4.0 would produce.
         # This enables this example to be run as a test under bazel 5.4.0.
         """sed -e 's#@//#//#'""",
-        """sed 's#"@python39_.*//:bin/python3"#interpreter#' >$@""",
-    ]),
+        """sed 's#"@python39_.*//:bin/python3"#interpreter#'""",
+        # Stick some buildifiler disables in as needed
+        """sed 's/^def /# buildifier: disable=function-docstring\\\ndef /g'""",
+    ]) + " >$@",
 )
 
 write_file(
@@ -50,3 +54,13 @@ diff_test(
     file1 = "requirements.bzl",
     file2 = ":make_platform_agnostic",
 )
+
+py_test(
+    name = "test",
+    srcs = ["test.py"],
+    deps = [
+        requirement("oletools"),
+        requirement("pcodedmp"),
+        requirement("requests"),
+    ],
+)

examples/pip_parse_vendored/WORKSPACE

@@ -22,6 +22,12 @@ load("@rules_python//python:pip.bzl", "pip_parse")
 pip_parse(
     name = "pip",
     python_interpreter_target = interpreter,
+    requirement_clusters = {
+        "vbap": [
+            "oletools",
+            "pcodedmp",
+        ],
+    },
     requirements_lock = "//:requirements.txt",
 )