fix: nodejs deprecated api <DEP0148>

[hyj1991]

Subpath folder mappings will be removed in a future release, so we should use direct subpath patterns instead.

The warning message is like below (with node-v16.9.0) when executing require('koa/lib/response');:

(node:80100) [DEP0148] DeprecationWarning: Use of deprecated folder mapping "./" in the "exports" field module resolution of the package at /Users/hyj1991/git/eggjs/egg-core/node_modules/koa/package.json.
Update this package.json to use a subpath pattern like "./*".
(Use `node --trace-deprecation ...` to show where the warning was created)

This PR is to be compatible with the omitted specifier path with require after fixing DEP0148:

require('koa/lib/application');
require('koa/lib/application.js');

require('koa/lib/response');
require('koa/lib/response.js');

require('koa/lib/request');
require('koa/lib/request.js');

require('koa/lib/context');
require('koa/lib/context.js');

require('koa/package.json');
require('koa/package');

Refs:

• https://nodejs.org/api/deprecations.html#DEP0148
• https://nodejs.org/api/packages.html#packages_subpath_folder_mappings

[codecov]

Codecov Report

Merging #1585 (1276f22) into master (2400985) will not change coverage.
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@@           Coverage Diff           @@
##           master    #1585   +/-   ##
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  Coverage   99.60%   99.60%           
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  Files           5        5           
  Lines         507      507           
  Branches      142      142           
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  Hits          505      505           
  Misses          2        2           

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[3imed-jaberi]

LGTM +1