refactor(internal/semver): move legacysemver and update imports

Move the `internal/legacylibrarian/legacysemver` package to
`internal/semver`, since we plan to continue using this logic for
version bumping.

Author: julieqiu

internal/legacylibrarian/legacylibrarian/commit_version_analyzer.go

@@ -22,7 +22,7 @@ import (
 
 	"github.com/googleapis/librarian/internal/legacylibrarian/legacyconfig"
 	"github.com/googleapis/librarian/internal/legacylibrarian/legacygitrepo"
-	"github.com/googleapis/librarian/internal/legacylibrarian/legacysemver"
+	"github.com/googleapis/librarian/internal/semver"
 )
 
 // getConventionalCommitsSinceLastRelease returns all conventional commits for the given library since the

internal/legacylibrarian/legacylibrarian/commit_version_analyzer_test.go

@@ -23,7 +23,7 @@ import (
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/googleapis/librarian/internal/legacylibrarian/legacyconfig"
 	"github.com/googleapis/librarian/internal/legacylibrarian/legacygitrepo"
-	"github.com/googleapis/librarian/internal/legacylibrarian/legacysemver"
+	"github.com/googleapis/librarian/internal/semver"
 )
 
 func TestShouldIncludeForRelease(t *testing.T) {

internal/legacylibrarian/legacylibrarian/release_stage.go

@@ -26,7 +26,7 @@ import (
 	"github.com/googleapis/librarian/internal/legacylibrarian/legacyconfig"
 	"github.com/googleapis/librarian/internal/legacylibrarian/legacydocker"
 	"github.com/googleapis/librarian/internal/legacylibrarian/legacygitrepo"
-	"github.com/googleapis/librarian/internal/legacylibrarian/legacysemver"
+	"github.com/googleapis/librarian/internal/semver"
 )
 
 type stageRunner struct {