Resolve git file paths to absolute before npm install

[aomarks]

Previously, an install error would occur if you specified a dependency substitution like this:

"my-dep": {
  "kind": "git",
  "repo": "../",
  "ref": "main",
  "setupCommands": [
    "npm i",
  ]
}

This happened because when repo is a relative path, it was getting evaluated as-is from the temp directory we generate for each install. Now, we first resolve such relative paths to absolute paths before install.

Fixes #202

cc @43081j

[cdata]

LGTM 👍