map: Prevent output destinations overlap with Caddyfile shorthands

[francislavoie]

Essentially, this tries to prevent configs like this:

example.com {
    map {host} {path} {
        example.com /example
    }
}

Where the output {path} is no good, because it's one of the placeholder shorthands. This means that the map destination would actually clobber an existing replacer output, and produce unexpected behaviour.

The implementation is kiiinda tricky though.

Since shorthand replacement happens before directive parsing (this surprised me when I was working on this, but "duh" I should've realized sooner), the map's Caddyfile parser actually sees {http.request.uri.path} instead of {path}; this means that we need to actually look for the replacements and map them back to the shorthand (for the error message).

With this change, the above Caddyfile will now spit out:

adapt: parsing caddyfile tokens for 'map': Caddyfile:2 - Error during parsing: destination {path} conflicts with a Caddyfile placeholder shorthand

Also, this adds {vars.*} shorthand. Nice to have now that we have a vars directive.

[mholt]

Kinda awkward, but probably good to prevent a foot-gun. Thanks Francis