Make use of pathlib.Path when dealing with paths within emscripten. NFC

[sbc100]

This change is very much like that large change I made to the test
suite. See #14191 and #14175.

It enables is to use unix style paths rather than arrays of path
components which has several advantages. As well as beeing more
readable and writable it also allows for things like git grep "musl/src" to work as expected. For an example of how much more
readable it makes the code see tools/system_libs.py.

This change was made almost exclusively with a bunch of sed
commands.

Because there is likely some minor performance overhead I did
some measurements.

Running ./tests/runner wasm2 --skip-slow:

before:
real 1m10.403s
user 49m40.135s
sys 6m14.466s
after:
real 1m10.397s
user 49m43.849s
sys 6m10.698s

Running ./embuilder build libc --force:

before:
real 0m5.597s
user 3m2.721s
sys 0m50.843s
after:
real 0m5.609s
user 3m0.998s
sys 0m49.796s

[dschuff]

I'm not familiar with pathlib, so I checked it out.
It looks like it doesn't have any abstraction for mac paths (they are just PosixPath), which is fine except that IIRC Mac filesystems are case-insensitive by default (whereas PosixPath treats them as case-sensitive). So, probably not a big problem if we already work OK on both Linux and Windows, but maybe something to watch out for.

[dschuff]

This is pretty nice and readable though.
as a side note, what would be the downside of just always using forward slash on windows, even without pathlib to canonicalize it? AFAIK, forward slashes just work fine on windows, and when I do random scripts or throwaway code I always just use / even on Windows

[sbc100]

This is pretty nice and readable though.
as a side note, what would be the downside of just always using forward slash on windows, even without pathlib to canonicalize it? AFAIK, forward slashes just work fine on windows, and when I do random scripts or throwaway code I always just use / even on Windows

I don't know what the major downsides would be (bit I imagine there must be some?), but I like the pathlib is consistent with what we do today (os.path.join). Keeps this change NFC for sure.

[sbc100]

I guess its nice for windows users when they see pathnames in log files or error message, or otherwise echo'd back at them if they look like windows paths.