Restructure this library to better separate low-level and high-level features

[pfmoore]

People seem to regularly expect this library to be a higher-level tool than it is. As a result, we get requests for functionality that we don't want to support, and users are disappointed that features that do exist don't meet their expectations.

I suggest that this library should be reorganised, and potentially split up, to better reflect different features.

1. A very low-level library, that has no external dependencies, that just handles calling backend hooks. Essentially, this would just be the wrappers.py file and support modules.
2. A slightly higher level library that depends on a particular toml library, and parses pyproject.toml and calls the backend. It should probably also return the build dependencies. I'm not 100% sure this is useful, but it would satisfy the needs of people wanting to manage dependencies themselves, but still be able to call something that does all the steps of building a project - see for example Don't pip install without user knowledge and allow to opt out #81.
3. Maybe a build environment management library - but my feeling is that getting such a thing right is pretty complex. I'd be inclined to say that if we were to provide a library interface to build environment management, it should be based on pip's implementation, not on the existing code.
4. A command line tool doing what is currently pep517.build, etc. Personally, this isn't something I particularly want to touch, but the growing popularity of pep517.build suggests that there's a demand for this.

I'd suggest that this project focus on being a pure library, covering (1) and (2) initially, with (3) a longer-term goal.

The command line functionality can be spun off into an independent project. I'd argue that the current build environment management code should probably go with it - it's not really robust enough to be a usable library at the moment, but it's needed for the command line tools.

[pradyunsg]

I'd say #83 solves this. :)

[layday]

pep517 is used extensively as a build front-end - whether the authors intended for it or not - for lack of PEP517 front-ends. What alternative is there following this change?

[pfmoore]

https://pypi.org/project/build/

[layday]

Thanks. It appears you have to install from source for the time being since the version on PyPI is not able to handle build requires.

[FFY00]

Yes, the current version does not build in isolation. That behavior will change in the new version, it will be enabled by default and you will be able to disable it.