Use pypa/installer

[hoodmane]

@pradyunsg suggested that we might want to switch to using pypa/installer to perform package installations.

From looking at pypa/installer, I think this is a good idea.

@rth said:

We would prefer not to add too many runtime dependencies to micropip but in this case, maybe we can indeed vendor installer a bit similar to how pip vendors packages it needs. We are already using pypa/packaging as a runtime dependency.