support cascading import

[JoviDeCroock]

fixes #1008

It looks like esm.sh now has a new import when the file from https://esm.sh/preact@10.17.1 is initially loaded 😅 this amounted to us throwing an invalid URL for the block that is shielded with try catch in doing so it works correctly.

Example output of id, importer

/stable/preact@10.17.1/es2022/preact.mjs preact
/stable/preact@10.17.1/es2022/preact.mjs preact/hooks
/stable/preact@10.17.1/es2022/hooks.js preact/hooks
/stable/preact@10.17.1/es2022/preact.mjs stable/preact@10.17.1/es2022/hooks.js

all of these start with a slash but aren't valid URLs when combined.

[developit]

Ooh nice catch.