[Bug]: `VirtualFS.mapToBase` incorrectly converts a full stop to an empty string

[merceyz]

Self-service

• I'd be willing to implement a fix

Describe the bug

VirtualFS.mapToBase incorrectly converts a full stop (.) to an empty string causing mkdirSync('.') to fail

berry/packages/yarnpkg-fslib/sources/VirtualFS.ts

Lines 110 to 113 in 5869e59

	const resolvedRoot = VirtualFS.resolveVirtual(this.baseFs.resolve(PortablePath.dot));
	const resolvedP = VirtualFS.resolveVirtual(this.baseFs.resolve(p));
	return ppath.relative(resolvedRoot, resolvedP);

Originally reported by @missing1984 on Discord with this repro https://github.com/missing1984/yarn2-webpack-ts

To reproduce

await expect(packageJsonAndInstall({})).resolves.toBeTruthy();

await expect(
  yarn(`node`, `-e`, `require('fs').mkdirSync('.', {recursive:true})`)
).resolves.not.toContain("no such file or directory");

Environment

System:
  OS: Windows 10 10.0.19042
  CPU: (16) x64 AMD Ryzen 7 3700X 8-Core Processor
Binaries:
  Node: 16.4.2 - ~\AppData\Local\Temp\xfs-2ddaff2b\node.CMD
  Yarn: 3.0.0-rc.9.dev - ~\AppData\Local\Temp\xfs-2ddaff2b\yarn.CMD
  npm: 7.19.0 - C:\nodejs\npm.CMD

Additional context

No response

[yarnbot]

This issue reproduces on master:

Error: expect(received).resolves.not.toContain()

Received promise rejected instead of resolved
Rejected to value: [Error: Command failed: /usr/bin/node /github/workspace/scripts/actions/../run-yarn.js node -e require('fs').mkdirSync('.', {recursive:true})
internal/fs/utils.js:314
    throw err;
    ^

Error: ENOENT: no such file or directory, mkdir
    at Object.mkdirSync (fs.js:1009:3)
    at NodeFS.mkdirSync (/tmp/tmp-17c0N6xZfLakLD/.pnp.cjs:5575:24)
    at makeCallSync.subPath.subPath (/tmp/tmp-17c0N6xZfLakLD/.pnp.cjs:8497:26)
    at ZipOpenFS.makeCallSync (/tmp/tmp-17c0N6xZfLakLD/.pnp.cjs:8721:26)
    at ZipOpenFS.mkdirSync (/tmp/tmp-17c0N6xZfLakLD/.pnp.cjs:8496:17)
    at VirtualFS.mkdirSync (/tmp/tmp-17c0N6xZfLakLD/.pnp.cjs:5890:24)
    at PosixFS.mkdirSync (/tmp/tmp-17c0N6xZfLakLD/.pnp.cjs:5890:24)
    at URLFS.mkdirSync (/tmp/tmp-17c0N6xZfLakLD/.pnp.cjs:5890:24)
    at [eval]:1:15
    at Script.runInThisContext (vm.js:134:12) {
  errno: -2,
  syscall: 'mkdir',
  code: 'ENOENT'
}

]
    at expect (/github/workspace/.yarn/cache/expect-npm-24.8.0-8c7640c562-0c0da74930.zip/node_modules/expect/build/index.js:138:15)
    at module.exports (evalmachine.<anonymous>:4:7)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)
    at async /github/workspace/.yarn/cache/@arcanis-sherlock-npm-2.0.2-91650a2501-627bee24a7.zip/node_modules/@arcanis/sherlock/lib/executeRepro.js:56:13
    at async executeInTempDirectory (/github/workspace/.yarn/cache/@arcanis-sherlock-npm-2.0.2-91650a2501-627bee24a7.zip/node_modules/@arcanis/sherlock/lib/executeRepro.js:17:16)
    at async Object.executeRepro (/github/workspace/.yarn/cache/@arcanis-sherlock-npm-2.0.2-91650a2501-627bee24a7.zip/node_modules/@arcanis/sherlock/lib/executeRepro.js:24:12)
    at async ExecCommand.execute (/github/workspace/.yarn/cache/@arcanis-sherlock-npm-2.0.2-91650a2501-627bee24a7.zip/node_modules/@arcanis/sherlock/lib/commands/exec.js:25:38)
    at async ExecCommand.validateAndExecute (/github/workspace/.yarn/cache/clipanion-npm-2.0.0-rc.16-b9444aaf89-91cf93ba72.zip/node_modules/clipanion/lib/advanced/Command.js:161:26)
    at async Cli.run (/github/workspace/.yarn/cache/clipanion-npm-2.0.0-rc.16-b9444aaf89-91cf93ba72.zip/node_modules/clipanion/lib/advanced/Cli.js:74:24)
    at async Cli.runExit (/github/workspace/.yarn/cache/clipanion-npm-2.0.0-rc.16-b9444aaf89-91cf93ba72.zip/node_modules/clipanion/lib/advanced/Cli.js:83:28)