Windows issues with filepath strings

[samaust]

I gave it a quick try on Windows. I found a few issues.

pkg-config/main.go

Line 37 in b10a85a

if strings.Contains(arg0, "/") {

This will contain "C:\" (or another letter) on Windows.

pkg-config/libs/flux/build.go

Line 394 in b10a85a

reModulePath := regexp.MustCompile(`/github\.com/influxdata\/flux@(v\d+\.\d+\.\d+.*)$`)

This will contain something like C:\installpath\github.com\influxdata\flux@v0.104.0

pkg-config/internal/modload/init.go

Line 52 in b10a85a

for cwd != "/" {

This will contain "C:\" on Windows. This last one causes an infinite loop.