x/mod/module: move pseudo-version functions from cmd/go/internal/modfetch

[jayconrod]

These functions in modfetch are used to handle pseudo-versions in the Go command. They're not tied to anything else in the go command and could be moved out into x/mod/module without much trouble.

func IsPseudoVersion(v string) bool
func IsZeroPseudoVersion(v string) bool
func PseudoVersion(major, older string, t time.Time, rev string) string
func PseudoVersionBase(v string) (string, error)
func PseudoVersionRev(v string) (rev string, err error)
func PseudoVersionTime(v string) (time.Time, error)
func ZeroPseudoVersion(major string) string

We could move SortVersions, too, while we're at it.

[gopherbot]

Change https://golang.org/cl/304150 mentions this issue: module: move pseudo-version functions into module

[gopherbot]

Change https://golang.org/cl/304229 mentions this issue: cmd/go: move pseudo-version code from modfetch to x/mod/module

[gopherbot]

Change https://golang.org/cl/304151 mentions this issue: semver: add ByVersion and Sort for sorting semantic version lists