Fall back to co-location heuristic if sourcemap url appears remote

[brettdh]

Applies a heuristic to guess that a sourcemap URL is a remote URL path (without scheme and domain):

1. The string starts with /
2. The string is not a path to an existing local file

Fixes #1870.

[github-actions[bot]]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[brettdh]

This is still on my todo list.

[brettdh]

...but the stalebot did not remove the label. 🤔

[szokeasaurusrex]

...but the stalebot did not remove the label. 🤔

I removed "Status: Stale" and added "Status: In Progress." Hopefully StaleBot leaves the PR alone going forward

[szokeasaurusrex]

@brettdh is this still in progress?

[brettdh]

Hmm, I thought I'd responded to these changes already, sorry. I'll try to push an update today.

[brettdh]

Responded to the comments above and resolved one. Please let me know if more is required.