Handle root module scenario in pub use transformer ; Improve hints when fails to parse a struct or enum

[fzyzcjy]

Changes

Related: #2102

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.12%. Comparing base (3716357) to head (1a56656).
Report is 4 commits behind head on master.

❗ Current head 1a56656 differs from pull request most recent head 9632a5f

Please upload reports for the commit 9632a5f to get more accurate results.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2124   +/-   ##
=======================================
  Coverage   99.12%   99.12%           
=======================================
  Files         485      485           
  Lines       19793    19800    +7     
=======================================
+ Hits        19619    19627    +8     
+ Misses        174      173    -1     

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