add --clearNimblePath; fixes #12601

[disruptek]

Per title; resets the list of Nimble search paths so that they may subsequently be manipulated by further nimblePath options.

[kaushalmodi]

Suggested change

	--clearNimblePath empty the list of Nimble package search paths
	--clearNimblePath empty the list of Nimble package search paths; more Nimble paths can be added after clearing using --NimblePath:PATH

[kaushalmodi]

Should the order of disableNimblePath and clearNimblePath be switched and have disableNimblePath call the other to do that setLen(0)?

[kaushalmodi]

How do we write tests for these combinations:

Switches	Effective Nimble paths
--noNimblePath	none
--NimblePath:abc	Default ~/.nimble + abc
--clearNimblePath --NimblePath:abc	abc
--NimblePath:abc --clearNimblePath	none
--NimblePath:abc --clearNimblePath --NimblePath:def	def
--NimblePath:abc --clearNimblePath --NimblePath:def --noNimblePath	none
--noNimblePath --NimblePath:abc --clearNimblePath --NimblePath:def	none

[genotrance]

I've not reviewed this change but am not a fan in principal since we have a goal of reducing Nim's awareness of Nimble in nim-lang/nimble#654. This adds even more behavior into Nim which will be harder to reduce or eliminate in a backwards compatible way.

[Araq]

@genotrance true but in the meantime people need a solution. However, the same behaviour could be added to the existing --noNimblePath.

[genotrance]

This was discussed and --clearNimblePath persisted over modifying existing behavior which exists for a legitimate use case.