feat: add graph subcommand to print dependency graph

[AkashKumar7902]

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

• N
• Y

solves: #263

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

kpm.go
pkg/cmd/cmd_graph.go
pkg/client.go

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

[coveralls]

Pull Request Test Coverage Report for Build 7946546745

Details

• -65 of 106 (38.68%) changed or added relevant lines in 2 files are covered.
• 17 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 40.915%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/client/client.go	41	48	85.42%
pkg/cmd/cmd_graph.go	0	58	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/client/client.go	17	56.58%

Totals
Change from base Build 7946161389:	-0.3%
Covered Lines:	1664
Relevant Lines:	4067

---

💛 - Coveralls

[AkashKumar7902]

I have implemented my own findSource function as the same is not provided by the library.

[AkashKumar7902]

@zong-zhe Please take a look

I have implemented two functions here Union and FindSource. The latter is implemented because the function or similar function is not provided by the libraray. The former is an override of the function provided by the library because I have assumed graph may be cyclic and error like "vertex already exist" & "edge already exist" needs to be ignored during cyclic graph construction.

Currently the graph is being built bottom-up in recursion , that's why we are getting the graph as a return value.
We can built the graph in top-down manner but that would require introducing an extra argument to the downloadDeps function. The top-down approach can decrease lines of code and overall complexity.

instead of using downloadDeps for providing two functionality, we can built a clone of downloadDeps function with the name say constructDepGrpah whose only task would be to construct the graph.

Please let me know of any changes.

[Peefy]

LGTM