[REG2.066a] Remained fix for issue 13098 - isValidFilename and alias this

[9rnsr]

https://issues.dlang.org/show_bug.cgi?id=13098

[mihails-strasuns]

LGTM

[9rnsr]

@MartinNowak Can you merge this?

[WalterBright]

Auto-merge toggled on

[denis-sh]

625ed56 isn't just a refactoring, it fixes these compilation errors probably caused by fixed IFTI:

std\path.d(545): Error: std.path.isRooted called with argument types (inout(char)[]) matches both:
std\path.d(1846):     std.path.isRooted!(inout(char)[]).isRooted(const(inout(char)[]) path)
and:
std\path.d(1856):     std.path.isRooted!char.isRooted(const(char)[] path)
std\path.d(1034): Error: template instance std.path.rootName!char error instantiating
std\path.d(1054):        instantiated from here: buildPath!(const(char[])[])
std\file.d(1943):        instantiated from here: buildPath!char