Add function to make paths absolute, which is different from canonicalization

[retep998]

Many users want to turn relative paths into absolute paths, but the only tool that libstd currently offers is canonicalize which is bad because it has to hit the filesystem which has several downsides:

1. It takes time to hit the filesystem.
2. The path has to actually exist.
3. It fails on certain types of drives such as RAM drives on Windows.
4. The path it creates is a \\?\ path which not all software can handle correctly and imposes requirements on further path manipulation that few users are even aware of.

Needing symbolic links actually resolved is an extremely rare use case, and is often misused to compare paths for equality when in reality you should be comparing file IDs due to things like hard links existing.

A new function (normalize or make_absolute or something, bikeshed away) should be added that will turn a relative path into an absolute path without touching the filesystem. On Windows this should either call GetFullPathNameW or do the pure Rust equivalent while on unixy platforms... something should happen, I have no idea.

If such a function did exist, rustc could start using that instead of canonicalize which would fix a whole host of issues including:
#74327
#74146
#59107
#58613
#55812
#52440
#48249
#45067
#42869

[Mark-Simulacrum]

Semi-related issues/PRs: #47402, #47363, and rust-lang/rfcs#2208.

[retep998]

One big question for the libs team to decide on is how to handle .. and . on linux. On Windows C:\foo\..\bar is equivalent to C:\bar and both paths will always point to the same thing. On linux however it is possible for /foo/../bar to not point to the same thing as /bar. Given a current directory of /foo and a relative path of ../bar, should normalize on linux return /foo/../bar or /bar?

[Xanewok]

Some prior art might be the Node path.normalize function which (it seems?) doesn't hit the FS and just tries to do the reasonable thing given path structure, e.g.:

For example, on POSIX:

path.normalize('/foo/bar//baz/asdf/quux/..');
// Returns: '/foo/bar/baz/asdf'

On Windows:

path.normalize('C:\\temp\\\\foo\\bar\\..\\');
// Returns: 'C:\\temp\\foo\\'

[nayeemrmn]

A function called normalize() will not be expected to convert to an absolute path, that name should be used for rust-lang/rfcs#2208.

[nayeemrmn]

I guess fs::canonicalize() is like realpath -e and the requested feature is like realpath -ms on Linux.

I want to reiterate that fs::normalize() should not be used for this, but for rust-lang/rfcs#2208 which is different from both of these. It doesn't account for CWD and leaves relative paths relative, making it the proper equivalent of Node's path.normalize() referenced above.