Fix crash with absolute path to binary artifact

[neonichu]

The code in Workspace.parseArtifacts() assumes prior validation of paths even though it is running before any such validation.

rdar://83059393

[neonichu]

@swift-ci please smoke test

[neonichu]

This is a kind of fun gap in our unit tests, because most of them run with a MockWorkspace or no Workspace at all, so they are never hitting this code.

I also think the existence of the non-validating path initializers is kind of an anti-pattern. Paths are almost always going to be user input in some fashion and this makes it way too easy to turn a missing validation into a runtime crash with no info.

[tomerd]

I also think the existence of the non-validating path initializers is kind of an anti-pattern. Paths are almost always going to be user input in some fashion and this makes it way too easy to turn a missing validation into a runtime crash with no info.

this sounds right to me - iow we should deprecate the non-validating initializer. @abertelrud wdyt?