[release-branch.go1.13] cmd/go: make module zip extraction more robust
    
    Currently, we extract module zip files to temporary directories, then
    atomically rename them into place. On Windows, this can fail with
    ERROR_ACCESS_DENIED if another process (antivirus) has files open
    before the rename. In CL 220978, we repeated the rename operation in a
    loop over 500 ms, but this didn't solve the problem for everyone.
    
    A better solution will extract module zip files to their permanent
    locations in the cache and will keep a ".partial" marker file,
    indicating when a module hasn't been fully extracted (CL 221157).
    This approach is not safe if current versions of Go access the module
    cache concurrently, since the module directory is detected with a
    single os.Stat.
    
    In the interim, this CL makes two changes:
    
    1. Flaky file system operations are repeated over 2000 ms to reduce
    the chance of this error occurring.
    2. cmd/go will now check for .partial files created by future
    versions. If a .partial file is found, it will lock the lock file,
    then remove the .partial file and directory if needed.
    
    After some time has passed and Go versions lacking this CL are no
    longer supported, we can start extracting module zip files in place.
    
    Updates #37802
    
    Change-Id: I467ee11aa59a90b63cf0e3e761c4fec89d57d3b6
    Reviewed-on: https://go-review.googlesource.com/c/go/+/221820
    Run-TryBot: Jay Conrod <jayconrod@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Michael Matloob <matloob@golang.org>
    Reviewed-by: Bryan C. Mills <bcmills@google.com>
    (cherry picked from commit 093049b3709eda7537ece92a2991918cf53782d6)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/223146