pkg_install: Add destdir attr & read rel paths. (#886)

Implementation notes:

Relative paths are interpreted against BUILD_WORKSPACE_DIRECTORY, not
BUILD_WORKING_DIRECTORY. This is for the following reasons:

The TODO tag explicitly convey the intention of using
BUILD_WORKSPACE_DIRECTORY for relative paths.

If destdir is specified in the attribute of the pkg_install() target,
interpreting it against BUILD_WORKSPACE_DIRECTORY is much stabler.
That is, no matter where your current cwd is, the destdir attribute
always refers to a path relative to the workspace root. For example:
```
pkg_install(name = "my_pkg_install", destdir = "out/dest")
```
```
cd <workspace_root>/<some_subdir>
bazel run //:my_pkg_install
```
This clearly conveys that the default destdir is
<workspace_root>/out/dest regardless of where the user runs the command.

The cost is that the --destdir command line argument becomes trickier
to understand. For example, if one is not familiar with pkg_install,
and below the workspace root they run:

```
cd <workspace_root>/out
bazel run //:my_pkg_install -- --destdir dest
```

They may expect the destdir to be set to <workspace_root>/out/dest; but
it is in fact `<workspace_root>/dest`.

We could also interpret the target attribute & the command line argument
separately (e.g. pkg_install(destdir_against_workspace)), but honestly
I think that's even more confusing when they interpret relative paths
differently. Please let me know if this is preferred by the maintainers.

Co-authored-by: HONG Yifan <elsk@google.com>