Support for multiple input source files

[fmease]

If multiple input source files are passed, we should do something very similar to cargo doc, i.e., “running rustc N-1 times and rustdoc N times” given N input files.

Figure out what -x/--cross-crate means if multiple files are passed. Probably reject.

We should have more “dev-y” configuration options available like passing --cfg doc to dependency crates (which is what Cargo doesn't do at the moment, CC rust-lang/rust#114952) or not running rustc at all (I don't know if that's useful, maybe for debugging cross-crate stuff; basically N input files => N rustdoc executions (modulo compiletest revisions, lol)).

[fmease]

Just hit a case where I need this. Plz fix!