[WIP] resolve: Make ambiguity checking stricter

[petrochenkov]

Testing a possible proper fix for the RustEmbed issue (#147196).
cc @LorrensP-2158466

[rustbot]

r? @jackh726

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[petrochenkov]

@bors try

[rust-bors]

💔 Test for 0e7f93c failed: CI. Failed jobs:

• try - dist-x86_64-linux (web logs, extended logs)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[petrochenkov]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 724e787 (724e78794d2cf06eef820a24c9a4608d63435e1d, parent: 2c0f4860cc86e2354a55ea04e738848168435c90)