Python: Conditional dependencies (environment markers)

Dear all,

Could support be added for conditional dependencies as available in Python
packages? See https://www.python.org/dev/peps/pep-0508/#environment-markers
for how it works in Python. I need this to convert Python packages with
conditional dependencies to Zero Install feeds. It's not necessary to
follow the spec exactly, just so long as I can convert to it.

If you are open to adding this feature to ZI, consider the initial design
below. I would love to hear about better approaches/naming for supporting
conditional dependencies. The design should be extensible as more special
cases will likely arise when converting other package types (e.g. conda) in
the future.

First, I would make the following interfaces:

- cpython.xml: CPython
- jython.xml: Jython
- ...
- python.xml: any python. This includes cpython.xml, jython.xml, ...

Then for python_version, python_full_version,
platform_python_implementation. I would convert to:

  <if selected='jython.xml' version='{constraint}'>
    <requires ...>

platform_machine could perhaps be expressed as:

  <if arch={converted_platform_machine}>
     <requires ...>

where arch refers to the arch of the actual system, not the implementation.
E.g. its value is never '*-*', always something concrete.

sys_platform (e.g. java1.8.0_51) could be handled like:

  <if selected='java.xml' version='1.8.0_51'>
    <requires ...>

Something more special would be required for platform_version as its value
depends on the runtime environment (number of cores, which kernel, ...):

  <if python:platform-version=...>

Or instead of trying to generalise, we could add each environment marker
variable as `<if python:{environment_variable_name} >`; but likely there is
interest for conditional dependencies in other languages as well.

I look forward to hearing your suggestions.
Cheers,

Tim

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