Issue Description:

It's just entirely broken. This example adapted from Foundation's test suite (NSURLWithString-parse-relative-basic-005):

let url = URL(string: "../joe/blow", relativeTo: URL(string: "http://example.com/foo/bar")!)!
print(url)                   // "../joe/blow -- http://example.com/foo/bar"
print(url.path)              // "/joe/blow"
print(url.standardized)      // "joe/blow -- http://example.com/foo/bar"
print(url.standardized.path) // "/foo/joe/blow"

What is happening here is that Foundation is standardising the relative portion of the path only, NOT the overall URL.

Typically, what "standardising" does is remove "." components, and other components which are popped by a subsequent "..". As the test shows, ".." components in the relative path portion may actually pop components from the base URL.

This is slightly similar to, but different from, https://bugs.swift.org/browse/SR-14145

'.standardized' is just far too naive in its whole approach. I don't expect these to be trivial fixes; the function may need to be substantially rewritten.