Description of problem:
When the %pyproject_buildrequires macro is used with the -w option (build wheel and read the metadata from it) and the build backend is already installed but pip isn't, then the build fails with:

Requirement satisfied: hatchling
   (installed: hatchling 1.13.0)
/usr/bin/python3: No module named pip
Traceback (most recent call last):
  File "/usr/lib/rpm/redhat/pyproject_buildrequires.py", line 532, in main
    generate_requires(
  File "/usr/lib/rpm/redhat/pyproject_buildrequires.py", line 444, in generate_requires
    generate_run_requirements(backend, requirements, build_wheel=build_wheel, wheeldir=wheeldir)
  File "/usr/lib/rpm/redhat/pyproject_buildrequires.py", line 352, in generate_run_requirements
    generate_run_requirements_wheel(backend, requirements, wheeldir)
  File "/usr/lib/rpm/redhat/pyproject_buildrequires.py", line 333, in generate_run_requirements_wheel
    raise RuntimeError('Failed to build the wheel for %pyproject_buildrequires -w.')
RuntimeError: Failed to build the wheel for %pyproject_buildrequires -w.


This is caused by the fact that the macro echoes pip but the %generate_buildrequires section does not restart before using it.


Version-Release number of selected component: 1.6.2-1


How reproducible: Always.

Steps to Reproduce:

1. Take e.g. https://src.fedoraproject.org/rpms/pyproject-rpm-macros/blob/rawhide/f/tests/python-userpath.spec

2. Apply this change:

 BuildRequires:  python3-devel
+BuildRequires:  python3-hatchling
...
 %generate_buildrequires
-%pyproject_buildrequires requirements-dev.txt -w
+%pyproject_buildrequires -w
...
 %check
-%pytest
+#pytest

(The requirements-dev.txt argument must be removed because it hides this problem.)

3. Build the spec in mock/koji.

Actual results:
/usr/bin/python3: No module named pip
...
RuntimeError: Failed to build the wheel for %pyproject_buildrequires -w.

Expected results:
pip is installed, package builds fine.


Additional info:
Possible fix: Before calling pyproject_wheel.build_wheel(...), requirements.add('pip', ...) and requirements.check(...).