Modularise the repository

[chrisnovakovic]

Issue #, if available: #1

Description of changes:

In PR #11, @bwagner5 refactored the code base as a Go module. Unfortunately, there were half a dozen other unrelated changes in the same PR, which have perhaps contributed to it not being merged. This PR is limited solely to the modularisation of the code, without the other changes that you may or may not want to accept.

• The first commit (4403e02) modularises the repository using the same directory structure as Restructure Repo w/ Go Modules and Add Darwin arm64 support #11. It targets the same version of Go as the one used by the Dockerfile (1.15). Support scripts, files and documentation have been updated to reflect the modularisation and removal of the vendor/ directory, even though this takes place in the following commit.
• The second commit (a413d1b) removes the vendored third-party code in vendor/. This is a separate commit because of how noisy the deletion of the vendorised code is compared with the changes made in the first commit.

make checkstyle fails, but it's currently failing on the mainline branch anyway (see #17), and it at least fails in the same way after these changes (in fact the new changes make it fail slightly less with Go 1.17). With checkstyle turned into a no-op in the Makefile, I was able to build working binaries with make build-linux-amd64 and a working Debian package with make package-deb-amd64. make quick-test behaves as expected, and passes.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[nijave]

Suggested change

	rm -rf build/* bin/ pkg/ .cover/
	rm -rf build/* bin/ .cover/

It looks like some files that shouldn't be cleaned up have been moved into pkg

[Yangtao-Hua]

Hi @chrisnovakovic,
Thank you for choosing session-manager-plugin and contribute to it. we already noticed it and some changes like the import path and upgrade to go-mod, these changes are meaningful and super useful, they are great reference for blocked customers. As it is a big change, we did not immediately merge it because need legal verification and some internal test.
Thanks again for the contribution!

Thanks,
Yangtao

[andymac4182]

@Yangtao-Hua Any idea when it will be merged or an answer as this would help enhance a lot of internal tooling that uses this tool.
Eg.
https://github.com/gjbae1212/gossm
https://github.com/tedsmitt/ecsgo