feat: add more information to 'kpm add' diagnostics.

[zong-zhe]

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

• N
• Y

fix #123

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

kpm/pkg/cmd

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 5689387539

• 21 of 151 (13.91%) changed or added relevant lines in 9 files are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.6%) to 39.721%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/cmd/cmd_init.go	0	1	0.0%
pkg/cmd/cmd_metadata.go	0	1	0.0%
pkg/env/env.go	0	2	0.0%
pkg/opt/opt.go	0	2	0.0%
pkg/settings/settings.go	4	8	50.0%
pkg/package/package.go	4	24	16.67%
pkg/mod/modfile.go	12	48	25.0%
pkg/cmd/cmd_add.go	0	64	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/cmd/cmd_add.go	2	0%
pkg/package/package.go	2	61.82%
pkg/mod/modfile.go	5	58.12%

Totals
Change from base Build 5688358543:	-0.6%
Covered Lines:	968
Relevant Lines:	2437

---

💛 - Coveralls