cmd/go: avoid accidental downgrades in 'go get' with latest and patch
Currently, 'go get -u' and 'go get -u=patch' avoid accidentally
downgrading modules by preventing upgrades in two cases:
1) If the current version is a prerelease that is semantically later
than the "latest" or "patch" version.
2) If the current version is a pseudoversion that is chronologically
newer than the "latest" or "patch" version.
With this change, 'go get m@latest' and 'go get m@patch' prevent
downgrades using the same checks.
Also: 'go get m@patch' now works if m is a module path but not a
package path (i.e., there is no package in the module root directory).
Fixes #30634
Fixes #32537
Change-Id: I916630c385b5f3ba7c13e0d65ba08f73a1a67829
Reviewed-on: https://go-review.googlesource.com/c/go/+/180337
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com>