Deno: [Feature Request] Support partial reload

[Fenzland]

There is a simple project with only few of files, but huge number of dependences such as deno_std. Now, the project has updated. I need to update my local caches by running:

deno fetch -r https://my.project.host/app.js

The trouble is the deno will re-download all dependences.

If the --reload option support parameters for specifying which host or hosts need to update:

> deno fetch --reload=https://my.project.host https://my.project.host/app.js

> deno fetch \
  --reload=https://my.project.host \
  --reload=https://some-host.need-update.too \
  https://my.project.host/app.js

We can save lots of time.

[bartlomieju]

It is a good first issue in Rust.

Roughly what needs to be done:

1. Configure reload CLI flag to match allow-read
   https://github.com/denoland/deno/blob/718c8c4360b5c42d1c0bcd97d7348908d915212e/cli/flags.rs#L57-L63
2. Add new field to DenoFlags, eg. cache_blacklist, again look up example with read_whitelist
   https://github.com/denoland/deno/blob/718c8c4360b5c42d1c0bcd97d7348908d915212e/cli/flags.rs#L32
3. Add this new cache_blacklist field to SourceFileFetcher and it's new method
   https://github.com/denoland/deno/blob/718c8c4360b5c42d1c0bcd97d7348908d915212e/cli/file_fetcher.rs#L69-L75
4. Handle blacklist appropriately in SourceFileFetcher::get_source_file_async and SourceFileFetcher::fetch_remote_source_async - ie. add some URL pattern matching

This flag should support only remote files (http/https, because local files are always fetched from disk and not cached internally).

[mhvsa]

currently working on it