Misc file tweaks

[TwitchBronBron]

Migrated from #329 but targeting v1 branch instead of master.

Notable changes:

• rename all instances of pathAbsolute to srcPath for brevity and clarity
• pkgPath strings now contain an actual pkg path (i.e. "source/main.brs" is now "pkg:/source/main.brs")
• pkgPath originally stored the platform-specific path separator (\ on windows, / on everything else). Now pkgPath always stores the unix separator because that's what is used on the roku device.
• combine some of the program file functions to auto-detect the path type and use the correct file collection (this.files or this.pkgMap) accordingly.
• program.files and program.pkgMap now store their keys in all lower case, which removes the need to iterate the collection to find files
• Add a normalizePath: boolean parameter to program.getFile to support skipping util.standardizePath when the caller knows the path has already been normalized