Remove the leading . (CurrentDir) prefix while normalizing

[schneems]

Today, if you pass ./a/b and a/b through normalize equality will tell you they are different. That was surprising to me.

Here is how other languages behave with the current Rust behavior for comparison:

Implementation	a	./a	a/	(empty)	.
Go path.Clean	a	a	a	.	.
Java Path.normalize	a	a	a	(empty)	(empty)
Node path.posix.normalize	a	a	a/	.	.
C++ lexically_normal	a	a	a/	(empty)	.
Ruby Pathname#cleanpath	a	a	a	.	.
Python os.path.normpath	a	a	a	.	.
Rust normalize_lexically (main)	a	./a	a	(empty)	.

Rust is the only one that keeps the leading .. This PR strips it so ./a normalizes to a (a lone . is still preserved):

Implementation	a	./a	a/	(empty)	.
Rust normalize_lexically (proposed)	a	a	a	(empty)	.

This aligns the leading . case with other languages.

Related tracking issue for normalize_lexically unstable feature #134694.

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r? @jhpratt

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[jhpratt]

I'm going to wait for t-libs to weigh in on #159863, as I don't know what the intent of this API is.

[ChrisDenton]

One issue is that on Windows those can be different paths in special cases. For example:

// This opens the console input 
std::fs::File::open(r"CONIN$").unwrap();
// This attempts to open a file called `CONIN$`
std::fs::File::open(r"./CONIN$").unwrap();