Break mutual dependency between please_pex and test runners (#266)

The please_pex tool depends on various third-party modules for the
built-in test runner bootstraps, which in turn require please_pex in
order to be built. This circular dependency prevents the plugin from
being built entirely from source - see #265.

The root cause of this circular dependency is the presence of the entry
points in `//third_party/python:{behave,pytest}` - `python_wheel`
doesn't require please_pex per se, but the use of the `binary` or
`entry_points` parameters results in `python_binary` being used to build
the outputs, which does require please_pex in order to make them
executable. The entry points aren't actually necessary on these targets,
though, because the generated .pex files aren't executed directly - they
are just unzipped and concatenated onto the please_pex binary to form
the bootstrap archive containing the built-in test runners and their
transitive dependencies.

Remove the `entry_points` parameters from the behave and pytest targets,
and refactor the `//tools/please_pex` dependency tree so that the test
runners and all of their transitive dependencies are present when the
bootstrap archive is concatenated onto the main please_pex binary.

The bootstrap archive's contents remain the same, with the following
omissions:

- `.bootstrap/plz.py`
  - This is written by please_pex when creating an executable .pex - its
    presence isn't meaningful or required in the bootstrap archive.
- `.bootstrap/py/_vendored_packages/apipkg-2.0.0.dist-info/__init__.py`
- `.bootstrap/py/_vendored_packages/iniconfig-1.1.1.dist-info/__init__.py`
  - These empty files were created by please_pex when creating the
    executable .pex for pytest - their presence isn't meaningful or
    required in the bootstrap archive.

Fixes #265.

Author: chrisnovakovic

third_party/python/BUILD

@@ -232,7 +232,6 @@ python_wheel(
         "_pytest",
         "pytest",
     ],
-    entry_points = "pytest:main",
     hashes = ["1d881c6124e08ff0a1bb75ba3ec0bfd8b5354a01c194ddd5a0a870a48d99b002"],
     licences = ["MIT"],
     version = "7.4.2",
@@ -271,10 +270,6 @@ python_wheel(
     outs = [
         "behave",
     ],
-    entry_points = {
-        "behave": "behave.__main__:main",
-        "behave_test": "setuptools_behave:behave_test",
-    },
     hashes = ["ebda1a6c9e5bfe95c5f9f0a2794e01c7098b3dde86c10a95d8621c5907ff6f1c"],
     licences = ["BSD-2-Clause"],
     version = "1.2.6",
@@ -417,31 +412,31 @@ python_wheel(
 
 # This is the minimum set of third-party packages required for the built-in test runners to work
 # with "plz test" and "plz cover". All built-in test runners should include this as a source.
-filegroup(
+python_library(
     name = "test_bootstrap",
     deps = [
         ":coverage",
         ":portalocker",
     ],
 )
 
-filegroup(
+python_library(
     name = "unittest_bootstrap",
     deps = [
         ":test_bootstrap",
         ":xmlrunner",
     ],
 )
 
-filegroup(
+python_library(
     name = "pytest_bootstrap",
     deps = [
         ":pytest",
         ":test_bootstrap",
     ],
 )
 
-filegroup(
+python_library(
     name = "behave_bootstrap",
     deps = [
         ":behave",

tools/please_pex/BUILD

@@ -22,30 +22,30 @@ go_binary(
 genrule(
     name = "please_pex",
     srcs = {
-        "bootstraps": [
-            "//third_party/python:coverage",
-            "//third_party/python:portalocker",
-            "//third_party/python:pytest",
-            "//third_party/python:xmlrunner",
-            "//third_party/python:behave",
-        ],
         "main": [":pex_main"],
     },
     outs = ["please_pex"],
     binary = True,
     cmd = [
-        'for i in $SRCS_BOOTSTRAPS; do if echo "$i" | grep -q \'\\.pex$\'; then "$TOOL" x -o . "$i"; rm -f "$i"; fi; done',
+        'for i in third_party/python/*.pex.zip; do "$TOOL" x -o . "$i"; rm -f "$i"; done',
         "rm -f third_party/python/__*_main__.py*",
         # Have to make sure these exist.
         "touch third_party/__init__.py third_party/python/__init__.py",
         "touch tools/__init__.py tools/please_pex/__init__.py",
+        "mkdir -p .bootstrap",
         "mv third_party/python/* .bootstrap",
         # A little cleanup.
         "rm -rf third_party .bootstrap/xmlrunner/extra .bootstrap/coverage/htmlfiles .bootstrap/.*.pex.zip",
         '"$TOOL" z -d -i .bootstrap -o "$OUTS" --preamble_file $(location :pex_main)',
     ],
+    needs_transitive_deps = True,
     tools = [CONFIG.ARCAT_TOOL],
     visibility = ["PUBLIC"],
+    deps = [
+        "//third_party/python:behave_bootstrap",
+        "//third_party/python:pytest_bootstrap",
+        "//third_party/python:unittest_bootstrap",
+    ],
 )
 
 python_test(