Normative: Source Phase Imports rebased to Import Attributes

[guybedford]

This is the latest PR diff for the Stage 3 Source Phase Imports proposal, based to the import attributes updates on main.

This PR replaces https://github.com/tc39/ecma262/pull/3094, since work moved back into the specification repo since then with further changes which are brought back here.

All review feedback on the previous PR was formerly also upstreamed, so that all review comments should be addressed. This sync was previously done in https://github.com/tc39/proposal-source-phase-imports/pull/60 in April 2024, so included the latest comments.

To summarize the outstanding discussions from that PR here:

• There were comments about the consistency of the new ModuleRequest handling, which should all be resolved by the complete import attributes rebase since that introduces this record.
• An outstanding discussion on the abstract class constructor in https://github.com/tc39/ecma262/pull/3094#discussion_r1223430222 and https://github.com/tc39/ecma262/pull/3094#discussion_r1258941494 and https://github.com/tc39/ecma262/pull/3094#discussion_r1225709647

Formerly we had a review approval from @syg as well (https://github.com/tc39/ecma262/pull/3094#pullrequestreview-1523004517) down to some questions in the threads.

This PR forms the new base for all subsequent module harmony proposals going forward including Import Defer and the ESM Phase Imports proposals, as such it will be important to ensure it is maintained towards Stage 4 progression.

[guybedford]

Since previously, test-262 tests were added in https://github.com/tc39/test262/pull/3980, https://github.com/tc39/test262/pull/4252 and https://github.com/tc39/test262/pull/4277.