pkgx uninstall

[mxcl]

Fixes #752; Refs #788

[coveralls]

Pull Request Test Coverage Report for Build 6653102864

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 96.892%

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Totals
Change from base Build 6641472810:	0.007%
Covered Lines:	1386
Relevant Lines:	1423

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💛 - Coveralls

[mxcl]

I like the idea of the scripts themselves containing the uninstall logic since it means no database that may be deleted or corrupted separately.

However we will thus need to uninstall before updating anything that is already installed since we have no other way to return to a clean state first and the provides may vary between install invocations.

Basically needs more thought on how it should work

[yoroshikun]

To ship this earlier can we "whitelist" packages that have clear install path (simple binary) while others be blocked?
(eg. git, node)

[mxcl]

I can prioritize finishing this. I didn’t think it was that important for 2 reasons:

1. Uninstall is much less common an operation than install
2. It’s pretty trivial to do by hand, you just delete the relevant files in one directory, it's not nearly the complexity of a normal package manager uninstall command that realisitically could not be done by hand in 99% of cases.