Make reqsort its own thing

projects/reqsort/BUILD

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+py_binary(
+    name = "reqsort",
+    main = "__main__.py",
+    deps = [
+        py_requirement("click"),
+    ]
+)

projects/reqsort/README.md

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+# reqsort
+
+A `requirements.txt` formatter and sorter.

projects/reqsort/__main__.py

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+"""
+Platform independent sorting/formatting for requirements.txt
+"""
+
+import re
+
+import click
+
+
+REQ_PATTERN = re.compile(
+    r"(?P<pkgname>[a-zA-Z0-9_-]+)(?P<features>\[.*?\])?==(?P<version>[^\s;#]+)|(.*?#egg=(?P<eggname>[a-zA-Z0-9_-]+))"
+)
+
+
+SHITLIST = [
+    "pip",
+    "pkg-resources",
+    "setuptools",
+]
+
+
+def sort_key(requirement: str) -> str:
+    requirement = requirement.lower()
+    match = re.match(REQ_PATTERN, requirement)
+    sort_key = (
+        (match.group("pkgname") or match.group("eggname"))  # Get the match group
+        .replace("-", "")  # We ignore -
+        .replace("_", "")  # And _
+    )
+    return sort_key
+
+
+@click.command()
+@click.option("--execute/--dryrun", "execute", default=False)
+@click.argument("requirements")
+def main(requirements, execute):
+    """Given the path of a requirements.txt, format it.
+
+    If running in --execute, rewrite the source file with sorted contents and exit 0.
+
+    If running in --dryrun, exit 0 if --execute would produce no changes otherwise exit 1.
+
+    """
+
+    with open(requirements) as f:
+        lines = f.readlines()
+        f.seek(0)
+        # Preserve an initial "buffer" for equality testing
+        initial_buff = f.read()
+
+    # Trim whitespace
+    lines = [l.strip() for l in lines]
+
+    # Discard comments and shitlisted packages
+    lines = [l for l in lines if not l.startswith("#") and not sort_key(l) in SHITLIST]
+
+    # And sort, ignoring case explicitly
+    lines = sorted(lines, key=sort_key)
+
+    # And generate a new "buffer"
+    new_buff = "\n".join(lines) + "\n"
+
+    if new_buff != initial_buff and not execute:
+        exit(1)
+
+    else:
+        with open(requirements, "w") as f:
+            f.write(new_buff)
+        exit(0)
+
+
+if __name__ == "__main__":
+    main()