Easy file-tests macros

Some ideas for easier writing tests. Currently people do stuff like

- test -f $PREFIX/include/mamba/test.hpp  # [unix]
- if not %LIBRARY_PREFIX%\include\mamba\test.hpp exit 1 # [win]

which is super error prone and hard to get right.

Ideas for “files” macros:

• site-packages("mamba") -> checks that pythonX.X/site-packages/<mamba> exists and has contents (at least one init.py)
• pymodule("mamba") -> checks that pythonX.X/site-packages/<mamba> exists and has contents (at least one init.py)
• lib("libmamba", soversion="...") -> check that at least libmamba.so / libamamba.dylib is found in lib/, additionally check for so-versions that link to that, or for windows, check that bin/libmamba.dll and lib/libmamba.lib are there
• include("mamba") -> check that include folder include/mamba/ exists
• include("mamba/test.hpp") -> check that file include/mamba/test.hpp` exists in package
• `bin(“mamba”)

I am unsure about the following ones:

• cmake_find("mamba") -> make sure that find_package(“mamba”) would work
• pkgconfig("mamba") -> make sure that pkgconfig mamba works

There are probably some good ideas to support other languages and their install prefixes. I don’t know where R installs to?!

We can also use qualifiers to disallow any files outside those matched by the macros, except maybe files in /share or some other data-centric directory. That would give pretty strict control over the package content.

[mariusvniekerk]

We probably want to express this as something like

- exists:
    site-packages: 
      - mamba
    pymodule:
      - mamba
    file:
       - $LIBRARY_PREFIX/some/file
    glob:
      - "**/another/file"

This has the nice advantage that we can include it easily in our json schema and get completions / validations at the json schema level for correctness of the specifiction

[wolfv]

One other lint we could do when we have a more semantic way of testing for a shared library is to automatically strongly warn if there is no run-exports.