Allow installing dependencies in the global python environment instead of a target directory.
Rationale:
We are working in bundling HA in Linux Embedded images, using the yocto ecosystem.
Such images are minimal, do not contain a toolchain, and duplication needs to be avoided. Dependencies packages are provided in advance as much as possible. When HA works in a custom target directory it is not benefiting from the packages provided in advance in the global environment.
A blocking point was described in the following discussion.
The installation of netdisco triggered the installation of netifaces. This package needs compiling and the installation fails. The attempts to provide netifaces in advance have not been successful given the custom path at installing the dependencies. As the pip call(–target) does not look in the global and try to (re)solve the dependency, while it is already satisfied in the global environment.
For instance, the following patch solved the situation(temporally?), demonstrating the issue.