fix: missing translations of remote built-in extensions

[hyrious]

@BlackHole1 noticed that some translations are missing when develop in remote mode. After digging a bit, we found that only built-in extensions' NLS messages are missing. For example, the Source Control view (contributed by the built-in 'Git' extension) and built-in extensions' configurations are in English.

Despite that the remote server will download and install the corresponding language pack extension, it does not pick the language pack up because of 2 problems:

1. There's no nls.keys.json in the server build, causing clp/{hash}.{lang}/* failed to generate.
2. When scanning built-in extensions, the UI language was not passed correctly to the backend.

This PR fixes the 2 problems.

Note about problem 1: The original code Promise.all([mkdir, read keys, read messages]) will fail when the keys file is missing, but the mkdir task would succeed and leave an empty folder which prevents it regenerate in the next calling. So I changed the testing logic from the folder to the file in it.

[hyrious]

@microsoft-github-policy-service agree