Allow colons in $PATH environment variables

[camh-]

Importer.AppendSearchFromEnv() splits the value of an environment variable using os.SplitList which uses os.ListSeparator as the separator rune. This makes it impossible to have that rune as a character in a path element. On Unix-like OSes, ListSeparator is a colon which prevents having a netpath element with a port number, or an IPv6 address.

Rather than using SplitList, do the split ourselves and allow the ListSeparator to be escaped with a backslash.

We'll still keep netpaths (double-slash for https) so you don't always have to escape the colon in the URL you would otherwise have to, but at least this will now allow ports.