enum: check enum value alias uniqueness when define enum type in sheet or proto file

[wenchy]

• [ ] close #97
• [x] load: add empty-file-content unittest case

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 18 lines in your changes missing coverage. Please review.

Project coverage is 64.40%. Comparing base (fc92612) to head (28af0df). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xproto/enum.go	63.63%	5 Missing and 3 partials :warning:
xerrors/ecode.go	0.00%	7 Missing :warning:
internal/xproto/value.go	50.00%	1 Missing and 1 partial :warning:
xerrors/desc.go	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   64.36%   64.40%   +0.03%     
==========================================
  Files          69       68       -1     
  Lines        7566     7560       -6     
==========================================
- Hits         4870     4869       -1     
+ Misses       2125     2116       -9     
- Partials      571      575       +4     

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