Store plugin requirements

[LecrisUT]

This is primarily for tmt but it doesn't hurt to implement it as a standard here.

Issue: Similarly to how tmt expanda upon an fmf tree, the tree might not be fully valid depending on how it is parsed, e.g. w.r.t. schema validation.

Approach: Along with .fmf/version, a .fmf/plugins could be optionally parsed. The format would be similar to that of a requirements.txt, i.e. PEP508, and parsed via packaging.requirements. So it may look like:

tmt
tmt-cmake [cmake] @ git+https://github.com/LecrisUT/tmt-cmake@main

Fmf responsibility here would be just to:

• Check that the plugin requirements are satisfied
  • name and specifier
  • maybe extras and marker
  • ignore url
• Print out a requirements.txt of the unsatisfied plugins when executing cli
• Print out all the .fmf/plugins as is (in case this will evolve later). Useful if the user would want to always install the plugins

Impact:

• tmt consumers can install external tmt plugins before tmt command execution
• fmf tree can be fully validated based on its syntax, e.g. is it a tmt-flavored tree, vanilla or something else?