Ensure absolute lockfile remotes for gems nested inside project

[vinistock]

Motivation

I discovered this problem while working on #2774 and I frankly don't understand how it wasn't surfaced before.

We were checking for a leading dot to perform remote relative path correction, but when you have gems nested inside a project, the relative path may not necessarily begin with a dot.

For example, if you have this in your Gemfile

gem "nested", path: "gems/nested"

Then the remote won't necessarily be ../gems/nested. It might just be gems/nested, which is also a relative path. We need to convert those into absolute paths too, otherwise setting up the composed bundle fails.

Implementation

I started using URI to check if the remote has a scheme. If it has a scheme, like http, then we don't want to perform any corrections.

If it has no scheme, then it's a file path and we can turn it into absolute.

Automated Tests

Added a test that doesn't pass before the fix.