nyc instrument should resolve source filenames.

[coreyfarrell]

Create ./somefile.js then run nyc instrument somefile.js. The instrumented code produces coverage data for the filename 'somefile.js' when it really should be producing coverage for path.resolve('somefile.js'). This causes oddness in reporting when combining pre-instrumented code with code that nyc instruments on the fly (during execution).

CC @AndrewFinlay

[AndrewFinlay]

Good to see this issue resolved, it was biting me in the instrument --cwd changes. I had to jump through hoops to get that test to pass. Dropping that requirement should make for a much cleaner implementation.