mod/modfile: accept language versions after v0.8.0-alpha.0

Currently, the earliest supported version is v0.8.0, but some people
were using the first modules experiment alpha versions just before that,
so allow versions from v0.8.0-alpha.0 instead.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ib16658bf8a54c3e35483318a12d4659939a5bb6c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194685
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

mod/modfile/modfile_test.go

@@ -39,12 +39,12 @@ var parseTests = []struct {
 	parse:    Parse,
 	data: `
 module: "foo.com/bar@v0"
-language: version: "v0.8.0"
+language: version: "v0.8.0-alpha.0"
 `,
 	want: &File{
 		Module: "foo.com/bar@v0",
 		Language: &Language{
-			Version: "v0.8.0",
+			Version: "v0.8.0-alpha.0",
 		},
 	},
 	wantDefaults: map[string]string{
@@ -458,7 +458,7 @@ language: {
 }
 
 func TestEarliestClosedSchemaVersion(t *testing.T) {
-	qt.Assert(t, qt.Equals(EarliestClosedSchemaVersion(), "v0.8.0"))
+	qt.Assert(t, qt.Equals(EarliestClosedSchemaVersion(), "v0.8.0-alpha.0"))
 }
 
 func TestLatestKnownSchemaVersion(t *testing.T) {

mod/modfile/schema.cue

@@ -39,7 +39,7 @@ versions: [string]: {
 	#Strict!: _
 }
 
-versions: "v0.8.0": {
+versions: "v0.8.0-alpha.0": {
 	// Define this version in terms of the later versions
 	// rather than the other way around, so that
 	// the latest version is clearest.