feat: supports add submod by ModSpec

[zong-zhe]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

2. What is the scope of this PR (e.g. component or file name):

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

[Peefy]

LGTM

[coveralls]

Pull Request Test Coverage Report for Build 11815907558

Details

• 3 of 87 (3.45%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 42.128%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/client/run.go	0	1	0.0%
pkg/visitor/visitor.go	1	2	50.0%
pkg/downloader/utils.go	0	82	0.0%

Totals
Change from base Build 11812932471:	-0.4%
Covered Lines:	4014
Relevant Lines:	9528

---

💛 - Coveralls