Ft/handle external pkg name in response (deepmap#752)

* feat: add test case issue-removed-external-ref

* chore: go generate

* feat: fix broken pkg name

* fix: broken generated code of strict-interface.tmpl for external schema

* fix: broken strict-interface.tmpl

* Update generated code

    generate ./...

And update user code

* ./internal/test/strict-server/chi/server.go
* ./internal/test/strict-server/gin/server.go
* ./internal/test/strict-server/echo/server.go

Co-authored-by: Marcin Romaszewicz <47459980+deepmap-marcinr@users.noreply.github.com>

internal/test/issues/issue-removed-external-ref/config.base.yaml

@@ -0,0 +1,12 @@
+---
+package: spec_base
+generate:
+  chi-server: true
+  strict-server: true
+  models: true
+import-mapping:
+  spec-ext.yaml: "github.com/deepmap/oapi-codegen/internal/test/issues/issue-removed-external-ref/gen/spec_ext"
+output: gen/spec_base/issue.gen.go
+output-options:
+  skip-prune: true
+    # skip-fmt: true

internal/test/issues/issue-removed-external-ref/config.ext.yaml

@@ -0,0 +1,10 @@
+---
+package: spec_ext
+generate:
+  chi-server: true
+  strict-server: true
+  models: true
+output: gen/spec_ext/issue.gen.go
+output-options:
+  skip-prune: true
+   # skip-fmt: true

internal/test/issues/issue-removed-external-ref/doc.go

@@ -0,0 +1,4 @@
+package head_digit_of_httpheader
+
+//go:generate go run github.com/deepmap/oapi-codegen/cmd/oapi-codegen --config=config.ext.yaml spec-ext.yaml
+//go:generate go run github.com/deepmap/oapi-codegen/cmd/oapi-codegen --config=config.base.yaml spec-base.yaml