chore: enable always_include_developer_search_paths for all swift_library targets in external Swift packages (#911)

Background: SPM always includes the developer search paths for
compilation and linking. Now that `rules_swift` supports including these
for non-test targets, we will enable this feature.

- Use `always_include_developer_search_paths` for generated
`swift_library` targets.
- Remove check for `XCTest` imports. Previously, we would mark targets
as `testonly = True` if `XCTest` imports were detected. With the
`always_include_developer_search_paths` feature, this is no longer
necessary.

Closes #791.

Author: cgrindel

examples/nimble_example/MODULE.bazel.lock

@@ -14,6 +14,7 @@ swift_library(
     tags = ["manual"],
     deps = [
         "@swiftpkg_nimble//:Sources_Nimble",
+        "@swiftpkg_nimble//:Sources_NimbleObjectiveC",
         "@swiftpkg_quick//:Sources_Quick",
     ],
 )