By assuming virtualization environment won't change during a the program's runtime, we can cache common/common_linux.Virtualization() with a simple map to reduce amount of system calls. I first mentioned this issue at https://github.com/shirou/gopsutil/pull/890#issuecomment-690211919

This reverts commit f58b2e36, reversing
changes made to 84e62157.

gopsutil is a transitive dependency of another project that I am integrating
into an internal build system. We target multiple platforms and as a part
of the build system for the large internal repo, we calculate the build
graph used to determine what targets have changed and need to be build /
tested as a single DAG for all platforms.

gopsutil currently does not form a DAG if linux and any other platform are
considered at the same time. linux is the only platform where the process
package imports the host package.

To remove this cycle, the relevant methods have been moved to internal/common
with the linux build tag and are consumed the host and process packages.

exec.LookPath searches for the file in the $PATH, which mean giving it an absolute path is against it's own purposes.

Fixes #415

NumProcs() just counts files under f, so use f.Readdirnames to avoid `lstat` syscall inside f.Readdir.

Package common wasn't used for public functions. Place it in an
internal directory to prevent other packages from using.

Remove the distributed references to "HOST_PROC" and "HOST_SYS"
consts and combine into a common function. This also helps so that
if a env var is defined with a trailing slash all will continue to
work as expected.

Fixes #100