Caused by https://github.com/dlang/dmd/pull/13134
Old behaviour
---
(stable) $ dmd foo.c
Error: module `foo` is in file 'foo.c' which cannot be read
---
New behaviour
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(stable) $ ./generated/linux/release/64/dmd foo.c
Error: unable to read module `foo`
Expected 'foo.c' or 'foo/package.d' in one of the following import paths:
import path[0] = ./generated/linux/release/64/../../../../../druntime/import
import path[1] = ./generated/linux/release/64/../../../../../phobos
(stable) $ mkdir bar
(stable) $ touch bar/package.d
(stable) $ ./generated/linux/release/64/dmd bar.d
Error: unable to read module `bar`
Expected 'bar.d' or 'bar/package.d' in one of the following import paths:
import path[0] = ./generated/linux/release/64/../../../../../druntime/import
import path[1] = ./generated/linux/release/64/../../../../../phobos
---