Only invoke Deno.mkdir() if the cache file does not exist

[TooTallNate]

This fixes using the prepare() function when the file
system is read-only (i.e. within AWS Lambda environment).

[manyuanrong]

It seems that there is no essential difference, are you sure this can solve the problem?

[TooTallNate]

Sorry, I perhaps didn't explain properly. The read-only filesystem's contents have been previously generated from a build step. The contents of a built zip file are extracted in the lambda runtime environment before making the filesystem be read-only.

So in this case, the cached plugin file will already be present in the .deno_plugins directory, thus causing the exists() check to pass, and making this mkdir() invocation not happen in that case (which avoids the EROFS error at runtime).

Let me know if that helps explain a bit better 😄

[manyuanrong]

got it

[manyuanrong]

Included in the v0.8.0 release