Enable modules and remove vendor directory

[ear7h]

This was pretty straight forward but there is one issue of vendoring the ci tools. This was something we did before but, if we want to vendor them (and track versions etc.) we also need to vendor the the normal dependencies, which we did not do before. If we want the same behavior as before, I think we should "vendor" the tools in the same way as rtreego. Please advise.

[coveralls]

Pull Request Test Coverage Report for Build 442

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-2.5%) to 55.34%

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Files with Coverage Reduction	New Missed Lines	%
planar/triangulate/delaunay/subdivision/subdivision.go	1	45.71%
<!--	Total:	1		-->

Totals
Change from base Build 440:	-2.5%
Covered Lines:	5819
Relevant Lines:	10515

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💛 - Coveralls

[gdey]

It looks like go mod will track the version's of the ci tools we are using. As long as we make sure to add them to the /internal/tools/tools.go file.

So, I think we are good on that part.