[core] Support leading `/` in artifact metadata lookup regexes.
[core] Support leading `/` in artifact metadata lookup regexes.

This change slightly augments the pre-existing regular expressions to
look for an optional `/` character at the start of the path entry,
meaning that both:

* `hab/pkgs/..../IDENT`
* `/hab/pkgs/..../IDENT`

will be found in Habitat Artifacts. This change will support any future
Windows-created packages using the 7zip program.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat