Maturin's toml parser errors out on dotted keys in Cargo.toml when building sdists

[bearcage]

It looks like something in the sdist build process for Maturin doesn't understand dotted keys in toml files.

Apologies, but I'm on-call this week so I won't be able to dig through Maturin to see what/where for a little while, hence making a bug report issue instead of a fix PR.

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Issue Template Q&A

Please provide the following information:

• Your python version (python -V): N/A (abi3, PYO3_NO_PYTHON)
• Your pip version (pip -V): N/A (same as above)
  • If this isn't the latest pip version, have you checked using the latest pip version? N/A
• The version of the bindings you're using, if any (e.g. pyo3, rust-cpython or cffi): Default
• Does cargo build work? Yes.
• If on windows, have you checked that you aren't accidentally using unix path (those with the forward slash /)? Repro'd on macos and linux.

Please list the exact steps required to reproduce your error with all command output and if possible with a repository:

• Create a new repository with two library crates in it:

demo/
  lib1/
    Cargo.toml
    src/
      lib.rs
  lib2/
    Cargo.toml
    src/
      lib.rs

• Make the lib1 crate refer to the other by path using dotted keys, like so:

[dependencies]
lib2.path = "../lib2"

• Maturin will fail with an error like the following (library name changed in this copypaste to match example above / redact internal repo names from my dayjob):

12:02:12  🔗 Found pyo3 bindings with abi3 support for Python ≥ 3.6
12:02:12  🐍 Not using a specific python interpreter (With abi3, an interpreter is only required on windows)
12:02:13  💥 maturin failed
12:02:13    Caused by: Failed to build source distribution
12:02:13    Caused by: Failed to add local dependency lib2 at /build/lib2 to the source distribution
12:02:13    Caused by: Failed to parse Cargo.toml at /build/lib1/Cargo.toml
12:02:13    Caused by: TOML parse error at line 23, column 5
12:02:13     |
12:02:13  23 | lib2.path      = "../lib2"

[bearcage]

It looks like this limitation comes from the upstream toml_edit:

• Issue: add support for toml 1.0 toml-rs/toml#58
• This diff (which looks to have stalled for a couple months) should fix it: Support dotted keys toml-rs/toml#73

[messense]

@konstin I don't think we really need a format preserving toml editing tool to do sdist, maybe we should just switch to the toml crate instead.