fix(macros-core): only track paths rustc can checksum

[varunshahdev]

Problem

With --cfg sqlx_macros_unstable, MtimeCacheBuilder::add_path passes every watched path to proc_macro::tracked::path. Some of those paths are not files:

• In query/metadata.rs::load_env, each directory searched for .env that has no .env is registered as the directory itself. fix(macros): don't look for .env outside workspace #4282 bounds that walk to the workspace, but it still registers directories.
• <manifest_dir>/sqlx.toml is registered even when it doesn't exist.

rustc can't checksum a directory or a missing file. Under cargo's -Z checksum-freshness it logs failed to compute checksum, omitting it from dep-info ... Is a directory (os error 21). The resulting dep-info entry has no checksum, so cargo considers the crate permanently dirty: it is recompiled on every build, even a no-op one.

That blocks the main reason to combine sqlx_macros_unstable with checksum-freshness: letting cargo rebuild a crate exactly when one of its .sqlx/query-*.json files changes.

Fix

Call proc_macro::tracked::path only when path.is_file(). The in-process MtimeCache still records every path, so its own invalidation is unchanged. The only cost is that rustc is no longer told about a .env or sqlx.toml that doesn't exist yet, so creating one later won't by itself trigger a rebuild. Before this change that case caused a rebuild on every build anyway.

Verification

I tested a single-crate project with one query!, built offline (SQLX_OFFLINE=true), using RUSTFLAGS="--cfg sqlx_macros_unstable" and CARGO_UNSTABLE_CHECKSUM_FRESHNESS=true on a 2026-09-22 nightly, with sqlx-macros-core 0.9.0 patched via [patch.crates-io]:

	no-op rebuild	no-op rebuild	.sqlx JSON edited
0.9.0	Dirty (rebuilt)	Dirty (rebuilt)	Dirty
with this change	Fresh	Fresh	Dirty (rebuilt)

Without sqlx_macros_unstable, the patched code is compiled out and behaviour is unchanged.