Add a showcommands function to envsetup.

This is similar to the old `m showcommands`, but operates on specific
output paths instead and does not require a build aside from the
initial invocation of soong to generate the ninja files. The
`--regenerate` option is accepted that causes the ninja file to be
regenerated first, but this is not the default because it is slow.
Just note that without this flag results may be stale.

It unfortunately doesn't work for everything in out. For example,
`$OUT/system/lib/libc.so` won't work because it is a symlink that
isn't in the build graph, but `$OUT/system/lib/bootstrap/libc.so`
works.

Test: showcommands out/target/product/walleye/system/lib/libz.so
Bug: None
Change-Id: I4cfded3db994044870fc71ae4bcf9a69224b1e15