Bug 1723237: Use pip-compile directly from vendored location r=ahal

We've been installing `piptools` so that we can use the `pip-compile`
binary. However, we can just as easily do
`python -m piptools compile ...` instead, which also allows us to
skip a redundant install.

Differential Revision: https://phabricator.services.mozilla.com/D123240

build/common_virtualenv_packages.txt

@@ -86,6 +86,7 @@ pth:third_party/python/more_itertools
 pth:third_party/python/mozilla_version
 pth:third_party/python/pathspec
 pth:third_party/python/pep487/lib
+pth:third_party/python/pip_tools
 pth:third_party/python/pluggy
 pth:third_party/python/ply
 pth:third_party/python/py

python/mozbuild/mozbuild/test/test_vendor.py

@@ -28,8 +28,9 @@ def test_up_to_date_vendor():
             # Since VendorPython thinks "work_dir" is the topsrcdir,
             # it will use its associated virtualenv and package configuration.
             # Since it uses "pip-tools" within, and "pip-tools" needs
-            # the "Click" library, we need to make it available.
-            file.write("pth:third_party/python/Click")
+            # the "Click" library, we need to make them available.
+            file.write("pth:third_party/python/Click\n")
+            file.write("pth:third_party/python/pip_tools\n")
 
         # Copy existing "third_party/python/" vendored files
         existing_vendored = os.path.join(topsrcdir, "third_party", "python")

python/mozbuild/mozbuild/vendor/vendor_python.py

@@ -21,14 +21,7 @@ class VendorPython(MozbuildObject):
         self.log_manager.enable_unstructured()
 
         vendor_dir = mozpath.join(self.topsrcdir, os.path.join("third_party", "python"))
-
         self.activate_virtualenv()
-        pip_compile = os.path.join(self.virtualenv_manager.bin_path, "pip-compile")
-        if not os.path.exists(pip_compile):
-            path = os.path.normpath(
-                os.path.join(self.topsrcdir, "third_party", "python", "pip_tools")
-            )
-            self.virtualenv_manager.install_pip_package(path, vendored=True)
         spec = os.path.join(vendor_dir, "requirements.in")
         requirements = os.path.join(vendor_dir, "requirements.txt")
 
@@ -41,7 +34,10 @@ class VendorPython(MozbuildObject):
             # resolve the dependencies and update requirements.txt
             subprocess.check_output(
                 [
-                    pip_compile,
+                    self.virtualenv_manager.python_path,
+                    "-m",
+                    "piptools",
+                    "compile",
                     tmpspec,
                     "--no-header",
                     "--no-index",