Fixes scoped packages cache and their offline installation

[sharno]

Summary
Fixes #4266. I flattened the cache folder (no more nesting of scoped packages in a scope folder) and included the scope in the name of the folder in cache.

Test plan
I can add tests to integration for testing the issue with installing offline packages. But want to make sure that this solution makes sense first.

[buildsize]

This change will increase the build size from 10.26 MB to 10.26 MB, an increase of 912 bytes (0%)

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	886.52 KB	886.55 KB	28 bytes (0%)
yarn-[version].js	3.86 MB	3.86 MB	396 bytes (0%)
yarn-legacy-[version].js	4.01 MB	4.01 MB	396 bytes (0%)
yarn-v[version].tar.gz	891.26 KB	891.29 KB	36 bytes (0%)
yarn_[version]all.deb	667.97 KB	668.02 KB	56 bytes (0%)

[buildsize]

This change will increase the build size from 10.26 MB to 10.26 MB, an increase of 912 bytes (0%)

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	886.52 KB	886.55 KB	28 bytes (0%)
yarn-[version].js	3.86 MB	3.86 MB	396 bytes (0%)
yarn-legacy-[version].js	4.01 MB	4.01 MB	396 bytes (0%)
yarn-v[version].tar.gz	891.26 KB	891.29 KB	36 bytes (0%)
yarn_[version]all.deb	667.97 KB	668.02 KB	56 bytes (0%)

[agoldis]

@sharno I have noticed your PR and wonder if it can affect #5032? Any chance you take few minutes to take a look?

[blink1073]

Thanks for this @sharno! I prefer this approach to fix #4266 over the one I proposed in #4820.