refactor: move internal manifest package to internal/manifest (#590)

refactor: move manifest package out of internal/pkg

The internal/pkg/ directory is a generic catch-all that the codebase is
moving away from; new packages live directly under internal/. This change
relocates the manifest package to internal/manifest/ with no behavioral
change.

Extracted from #543 to keep that PR's diff focused on the two-way manifest
sync feature.

Author: mwbrooks

cmd/manifest/validate.go

@@ -23,7 +23,7 @@ import (
 	"github.com/slackapi/slack-cli/internal/app"
 	"github.com/slackapi/slack-cli/internal/cmdutil"
 	"github.com/slackapi/slack-cli/internal/iostreams"
-	"github.com/slackapi/slack-cli/internal/pkg/manifest"
+	"github.com/slackapi/slack-cli/internal/manifest"
 	"github.com/slackapi/slack-cli/internal/prompts"
 	"github.com/slackapi/slack-cli/internal/shared"
 	"github.com/slackapi/slack-cli/internal/shared/types"

internal/pkg/manifest/validate.go internal/manifest/validate.gointernal/pkg/manifest/validate.go renamed to internal/manifest/validate.go

@@ -25,7 +25,7 @@ import (
 	"github.com/slackapi/slack-cli/internal/config"
 	"github.com/slackapi/slack-cli/internal/experiment"
 	"github.com/slackapi/slack-cli/internal/icon"
-	"github.com/slackapi/slack-cli/internal/pkg/manifest"
+	"github.com/slackapi/slack-cli/internal/manifest"
 	"github.com/slackapi/slack-cli/internal/shared"
 	"github.com/slackapi/slack-cli/internal/shared/types"
 	"github.com/slackapi/slack-cli/internal/slackerror"