import from symbolic #bindirs, #libdirs, etc

[memsharded]

Changelog: Feature: imports functionality can import from "symbolic" names, preceded with @, like @bindirs, @libdirs, etc. This allows importing files from variable package layouts, including custom package_info() layouts (like cpp_info.bindirs = ["mybin"] can be used with src="@bindirs"), and editable package layouts

Docs: conan-io/docs#1547

Related: #4466

[jgsogo]

Something needed 👍 I would say it closes #5928 as it provides a much better way to solve the issue.

The only ther character I can think about is / because it is invalid in a directory/file name, and it also doesn't make sense at the beginning of the string. Another alternative could be cpp_info..

[jgsogo]

You can build a list of paths, and then avoid duplicating this code:

if src.startswith("#"):
    ...
    source_dirs = cpp_info_dirs
else:
    source_dirs = [src]

for source_dir in source_dirs:
    files = file_copier(....)
    ....

[memsharded]

Reviewed and added extra error checking test.

[lasote]

Merging, I need this. but please don't forget the docs