fix(installer): install hash-pinned dependencies and build tools (#856)

* fix(installer): install hash-pinned dependencies and build tools

* fix(installer): add cairo-devel for the pinned pycairo build

* fix(installer): reject missing or empty bootstrap exports

* fix(installer): preserve remote behavior for older tags

---------

Co-authored-by: Jatin K Malik <jatinkrmalik@gmail.com>

scripts/installer_requirements.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+"""Select committed requirement blocks without changing pins, markers or hashes.
+
+The installer uses complete exports for normal installs and just pywhispercpp's
+block for backend rebuilds. Missing exports or packages are errors, never an
+empty successful install. Only the standard library is needed before bootstrap.
+"""
+
+from __future__ import annotations
+
+import argparse
+import re
+from pathlib import Path
+
+
+def select_requirements(source: Path, package: str | None = None) -> str:
+    """Return an export, optionally retaining only one distribution's blocks."""
+    text = source.read_text(encoding="utf-8")
+    blocks = re.split(r"(?=^[A-Za-z0-9_.-]+==)", text, flags=re.MULTILINE)
+    requirements = blocks[1:]
+    if not requirements:
+        raise ValueError(f"No pinned requirements in {source}")
+    if package is not None:
+        normalize = lambda name: re.sub(r"[-_.]+", "-", name).lower()
+        requirements = [
+            block
+            for block in requirements
+            if normalize(block.split("==", 1)[0]) == normalize(package)
+        ]
+        if not requirements:
+            raise ValueError(f"No pin for {package} in {source}")
+    return blocks[0] + "".join(requirements)
+
+
+def main() -> None:
+    """Write the selected export to the installer's scratch directory."""
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("source", type=Path)
+    parser.add_argument("destination", type=Path)
+    parser.add_argument("--package")
+    args = parser.parse_args()
+    try:
+        text = select_requirements(args.source, args.package)
+    except (OSError, ValueError) as error:
+        parser.exit(1, f"Cannot install pinned dependencies: {error}\n")
+    args.destination.write_text(text, encoding="utf-8")
+
+
+if __name__ == "__main__":
+    main()