gh-37352: `sage --package create`: When re-creating with a different source type, clean up
    
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For example, when switching from a `pip` package to `wheel` package,
remove the `requirements.txt` file.
When switching from `normal` to `wheel`, remove the `spkg-install.in`
file.

One of these cases was handled by a commit on #36641, cherry-picked from
there.
Generalizing here to avoid mistakes such as
#37129 (comment) in
the future.

Example:
```
$ ./sage -package create imagesize --pypi
Downloading tarball from https://pypi.io/packages/py2.py3/i/imagesize/im
agesize-1.4.1-py2.py3-none-any.whl to /Users/mkoeppe/s/sage/sage-
rebasing/worktree-pristine/upstream/imagesize-1.4.1-py2.py3-none-any.whl
[......................................................................]
$ git status
On branch sage_package_create_remove_files
Your branch is ahead of 'upstream/develop' by 3 commits.
  (use "git push" to publish your local commits)

Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   build/pkgs/imagesize/SPKG.rst
        modified:   build/pkgs/imagesize/checksums.ini
        modified:   build/pkgs/imagesize/install-requires.txt
        deleted:    build/pkgs/imagesize/spkg-install.in
        modified:   build/pkgs/imagesize/type
```

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### ⌛ Dependencies

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URL: #37352
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

Author: Release Manager

build/sage_bootstrap/creator.py

@@ -80,6 +80,16 @@ def heading(title, char='-'):
             if upstream_contact:
                 f.write('{0}\n\n'.format(upstream_contact))
 
+    def _remove_files(self, files):
+        """
+        Remove ``files`` from the package directory if they exist.
+        """
+        for file in files:
+            try:
+                os.remove(os.path.join(self.path, file))
+            except OSError:
+                pass
+
     def set_python_data_and_scripts(self, pypi_package_name=None, source='normal'):
         """
         Write the file ``dependencies`` and other files for Python packages.
@@ -90,6 +100,8 @@ def set_python_data_and_scripts(self, pypi_package_name=None, source='normal'):
         If ``source`` is ``"wheel"``, write the file ``install-requires.txt``.
 
         If ``source`` is ``"pip"``, write the file ``requirements.txt``.
+
+        Remove existing files that belong to other source types.
         """
         if pypi_package_name is None:
             pypi_package_name = self.package_name
@@ -101,13 +113,23 @@ def set_python_data_and_scripts(self, pypi_package_name=None, source='normal'):
                 f.write('cd src\nsdh_pip_install .\n')
             with open(os.path.join(self.path, 'install-requires.txt'), 'w+') as f:
                 f.write('{0}\n'.format(pypi_package_name))
+            # Remove this file, which would mark the package as a pip package.
+            self._remove_files(['requirements.txt'])
         elif source == 'wheel':
             with open(os.path.join(self.path, 'install-requires.txt'), 'w+') as f:
                 f.write('{0}\n'.format(pypi_package_name))
+            # Remove this file, which would mark the package as a pip package.
+            self._remove_files(['requirements.txt'])
+            if pypi_package_name != 'pip':
+                # 'pip' should be the only wheel package that has a custom spkg-install.in script.
+                # Remove the script for all other wheel packages, to avoid errors when
+                # switching from normal to wheel packages.
+                self._remove_files(['spkg-build.in', 'spkg-install.in', 'spkg-install'])
         elif source == 'pip':
             with open(os.path.join(self.path, 'requirements.txt'), 'w+') as f:
                 f.write('{0}\n'.format(pypi_package_name))
+            self._remove_files(['checksums.ini', 'spkg-build.in', 'spkg-install.in', 'spkg-install', 'install-requires.txt'])
         elif source == 'script':
-            pass
+            self._remove_files(['checksums.ini', 'requirements.txt'])
         else:
             raise ValueError('package source must be one of normal, script, pip, or wheel')

src/doc/en/developer/packaging.rst

@@ -92,6 +92,10 @@ the following source types:
 
    - its version number is defined by the required file ``package-version.txt``;
 
+   - no build and install scripts are needed
+     (with one exception: the package :ref:`spkg_pip` installs itself from
+     its wheel using a custom install script);
+
    - Sage records the version number of the package installed using a file in
      ``$SAGE_LOCAL/var/lib/sage/installed/`` and will rerun the installation
      if ``package-version.txt`` changes.