feat: add '-E/--external' to api.

[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):

KCLVM/kclvm/api

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

add '-E/--external' to api.
kpm will tell kcl where to find external packages by api through '-E/--external'.

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

6. Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[Peefy]

LGTM

[coveralls]

Pull Request Test Coverage Report for Build 5188693491

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.988%

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Totals
Change from base Build 5173237368:	0.0%
Covered Lines:	2300
Relevant Lines:	2614

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 5188693493

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• 31 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-20.0%) to 68.035%

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Files with Coverage Reduction	New Missed Lines	%
compiler_base/session/src/lib.rs	31	50.0%

Totals
Change from base Build 5173237368:	-20.0%
Covered Lines:	30824
Relevant Lines:	45306

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💛 - Coveralls