dev: isolates the code borrowed from Go

[ldez]

Isolates the code borrowed from Go to internal/go/:
- moves package from internal/cache to internal/go/cache
- moves package from internal/mmap to internal/go/mmap
- moves package from internal/quoted to internal/go/quoted
- moves package from internal/robustio to internal/go/robustio
- moves package from internal/testenv to internal/go/testenv

Cuts direct dependency to internal/go/cache except for internal/pkgcache:

• accesses to the cache are made only through internal/pkgcache

Renames internal/pkgcache to internal/cache.

Updates the documentation history of the internal packages.

1 commit 1 change: it's easier to review the changes by commit because the moves and renaming confuse git diff.

Follows #5098