gh-92041: Avoid a second sys.modules scan in inspect.getmodule

[eendebakpt]

When inspect.getmodule() computes its filename cache key, getabsfile() consults getsourcefile(), which recurses into getmodule() — scanning sys.modules a second time on every cache miss. Cache hits also pay getsourcefile()'s linecache probe and os.path.exists() call. This PR passes the filename explicitly, so the key is computed without the recursion.

Benchmark

10,000 synthetic modules in sys.modules

Workload	main	PR	Speedup
Cached filename hit	4.13 µs	0.85 µs	4.9x
Unregistered frame (repeated miss)	3.19 ms	1.55 ms	2.1x
Bare code object (repeated miss)	3.06 ms	1.53 ms	2.0x
Miss after a registry mutation	3.14 ms	1.67 ms	1.9x
Function fast path	0.168 µs	0.168 µs	no change

Benchmark script

import inspect
import sys
import timeit
import types

for i in range(10_000):
    m = types.ModuleType(f"fake_{i}")
    m.__file__ = f"/tmp/fake/mod_{i}.py"
    sys.modules[m.__name__] = m

real_file = sys.modules["fake_1"].__file__
code_hit = compile("pass", real_file, "exec")
code_miss = compile("pass", "<generated>", "exec")
ns = {"sys": sys, "out": {}}
exec(compile("out['frame'] = sys._getframe()", "<generated>", "exec"), ns)
frame = ns["out"]["frame"]

def a_function():
    pass

inspect.getmodule(code_hit)  # warm the cache

for label, fn in [
    ("cached filename hit", lambda: inspect.getmodule(code_hit)),
    ("unregistered frame (miss)", lambda: inspect.getmodule(frame)),
    ("bare code object (miss)", lambda: inspect.getmodule(code_miss)),
    ("function fast path", lambda: inspect.getmodule(a_function)),
]:
    number, elapsed = timeit.Timer(fn).autorange()
    print(f"{label:28s} {elapsed / number * 1e6:10.3f} us")

• Issue: Slow inspect.getmodule #92041