Defer the eight imports that are not already pulled in by pdb's
required-eager imports (cmd, bdb, code, typing and the guarded
_pyrepl.utils), and lift five unguarded function-level imports to
module-scope lazy imports.

Guard Pdb._get_asyncio_task() on 'asyncio' in sys.modules. It runs on every
interaction(), so without the guard the deferred asyncio import is paid back
immediately on the first breakpoint and the lazy import buys nothing for an
actual debugging session. If asyncio was never imported there cannot be a
running task, so returning None is equivalent.

The guarded imports (readline, rlcompleter, _pyrepl) stay function-level:
they sit in try/except ImportError blocks, and hoisting them would move the
failure outside the guard.

import pdb              -42.1%
breakpoint() -> cont    -41.1%
python -m pdb script    -33.4%
pdb post-mortem         -33.3%

Drop the other lazy imports and the lifted function-level imports, leaving
just the asyncio deferral and the _get_asyncio_task() guard. Those two carry
most of the win and keep the diff to five lines.

import pdb              -28.8% (was -42.1% with the full set)
breakpoint() -> cont    -28.5%
python -m pdb script    -26.0%
pdb post-mortem         -24.6%

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