CLI/API: support a no-remote mode

[marktsuchida]

--no-remote should prevent all downloads.
Use the cached index; fail if not cached.
Fail if the matched JDK is not yet installed.

Note that this should not change the version matching rules: "17" should still request the latest 17.x in the index, without falling back to the latest that happens to be installed.

[marktsuchida]

See #6 for "latest among installed".

[marktsuchida]

Also an environment variable CJDK_NO_REMOTE=1.

[ctrueden]

Maven calls this "offline mode" with flag -o or --offline. I personally like that naming better than --no-remote just for succinctness.