use URL.Opaque when available

[coolaj86]

This change will cause all of the following to be handled correctly:

file:relpath/to/thing
file:/abspath/to/thing
file://relpath/to/thing
file:///abspath/to/thing

The parsed URL with have either u.Opaque or u.Path, never both.

A URL represents a parsed URL (technically, a URI reference).

The general form represented is:

[scheme:][//[userinfo@]host][/]path[?query][#fragment]

URLs that do not start with a slash after the scheme are interpreted as:

scheme:opaque[?query][#fragment]

[coveralls]

Pull Request Test Coverage Report for Build 242

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 57.728%

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Totals
Change from base Build 240:	0.0%
Covered Lines:	676
Relevant Lines:	1171

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 242

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 57.728%

---

Totals
Change from base Build 240:	0.0%
Covered Lines:	676
Relevant Lines:	1171

---

💛 - Coveralls

[dhui]

Thanks for the PR!