Modify package install_path based on build args

Hi

I'm trying to modify the `install_path` used in a `rezbuild.py` package so if I pass
the argument `dev` to `rez-build` like so: `rez-build --install --ba dev` the install folder get renamed to `../dev` instead of `version` found in the package.

Basically I want it to install package `foo` version `1.0.0` into the local package path but instead of install it into `1.0.0` folder I want `dev` folder name.

So I've to do this in `rezbuild.py`, but it seems that `rez-build` is doing something else to copy the `package.py` file to the default `install_path` with package version folder.

Here what I've done:

in `rezbuild.py`:

```python
def _rename_to_dev():
# override install_path version if DEV args is found
version_dir=os.path.basename(install_path)
dev_install_path=install_path.replace(version_dir,'dev')

print "Renaming %s to %s" %(install_path,dev_install_path)
os.rename(install_path, dev_install_path)
....
if "install" in (targets or []):
_install()

if "dev" in (build_args or []):
_rename_to_dev()
```

After the build install I correctly have my folder renamed to `dev` but it missing the `package.py` file
and this file has been put into the `1.0.0` folder version like so:

```shell
.../packages/builds/foo/1.0.0
contains only the package.py file
```

```
.../packages/builds/foo/dev
contains my deployed package files
```

Any idea on how I'm supposed to do this correctly ?

Thanks!
Maty

Start with the rez-build install flow and the rezbuild.py install handling described in the report, then trace where package.py is copied after the deployment is renamed. Reproduce the command `rez-build --install --ba dev` and verify that the deployed files and package.py are both under the dev directory, with no package.py left under 1.0.0.

Written by the indexing model from the issue text.