Skip filesystem syscalls in inspect for binary-bundled modules.

Modules bundled into an Arcadia binary (loaded via the ResourceImporter
in library/python/runtime_py3/__res.py) expose a relative __file__ such
as 'contrib/tools/python3/Lib/inspect.py'. There is no real file behind
that path, but inspect.getmodule() and inspect.getsourcefile() probe
the filesystem for it anyway:

  * getmodule() walks sys.modules and calls os.path.realpath() on each
    module's path. realpath() in turn calls os.lstat() for every path
    component (see posixpath.realpath), producing dozens to hundreds of
    syscalls per invocation. For a relative path getabsfile() first
    joins it with the current working directory, so the resolved path
    depends on where the binary was launched from -- often pointing at
    an unrelated file on disk.

  * getsourcefile() calls os.path.exists() on the same relative path,
    which is again resolved against CWD and triggers an os.stat().

Both probes are useless for bundled modules: their source is served by
the PEP 302 loader, not the filesystem. We restrict the realpath() and
exists() calls to genuinely absolute paths, which is the case for
modules loaded from disk via Y_PYTHON_SOURCE_ROOT (see __res.py: those
are passed to compile() with an absolute filename, while bundled
modules keep the Arcadia-relative path baked in at compile time).

linecache is already handled by fix-traceback.patch.

--- a/Lib/inspect.py	(index)
+++ b/Lib/inspect.py	(working tree)
@@ -980,7 +980,7 @@ def getsourcefile(object):
     # return a filename found in the linecache even if it doesn't exist on disk
     if filename in linecache.cache:
         return filename
-    if os.path.exists(filename):
+    if os.path.isabs(filename) and os.path.exists(filename):
         return filename
     # only return a non-existent filename if the module has a PEP 302 loader
     module = getmodule(object, filename)
@@ -1028,10 +1028,10 @@ def getmodule(object, _filename=None):
                 # Have already mapped this module, so skip it
                 continue
             _filesbymodname[modname] = f
-            f = getabsfile(module)
-            # Always map to the name the module knows itself by
-            modulesbyfile[f] = modulesbyfile[
-                os.path.realpath(f)] = module.__name__
+            absfile = getabsfile(module)
+            modulesbyfile[absfile] = module.__name__
+            if os.path.isabs(f):
+                modulesbyfile[os.path.realpath(absfile)] = module.__name__
     if file in modulesbyfile:
         return sys.modules.get(modulesbyfile[file])
     # Check the main module