BuildStream seems to need the repository which has been install from after installation

Background

[//]: # Installing buildstream from source using instructions but in a removable disk makes buildstream to fail to run

Task description

[//]: # Use case

• [ ] Clone the repository in USB/SDCard/external harddrive
• [ ] Install buildstream from source following the instructions in https://buildstream.gitlab.io/buildstream/install_main.html#installing-from-source-recommended
• [ ] Remove/Umount the USB/SDCard/external harddrive
• [ ] bst command will fail

Logs

$ bst
Traceback (most recent call last):
  File "/home/ubuntu/.local/bin/bst", line 5, in <module>
    from buildstream._frontend import cli
ModuleNotFoundError: No module named 'buildstream'

$ sudo mount /dev/sda /mnt

$ bst
Usage: bst [OPTIONS] COMMAND [ARGS]...

  Build and manipulate BuildStream projects

  Most of the main options override options in the user preferences
  configuration file.

Options:
  --version
  -c, --config PATH               Configuration file to use
  -C, --directory DIRECTORY       Project directory (default: current
                                  directory)
  --on-error [continue|quit|terminate]
                                  What to do when an error is encountered
  --fetchers INTEGER              Maximum simultaneous download tasks
  --builders INTEGER              Maximum simultaneous build tasks
  --pushers INTEGER               Maximum simultaneous upload tasks
  --network-retries INTEGER       Maximum retries for network tasks
  --no-interactive                Force non interactive mode, otherwise this
                                  is automatically decided
  --verbose / --no-verbose        Be extra verbose
  --debug / --no-debug            Print debugging output
  --error-lines INTEGER           Maximum number of lines to show from a task
                                  log
  --message-lines INTEGER         Maximum number of lines to show in a
                                  detailed message
  --log-file FILENAME             A file to store the main log (allows storing
                                  the main log while in interactive mode)
  --colors / --no-colors          Force enable/disable ANSI color codes in
                                  output
  --strict / --no-strict          Elements must be rebuilt when their
                                  dependencies have changed
  -o, --option OPTION VALUE       Specify a project option
  -h, --help                      Show this message and exit.

Commands:
  build          Build elements in a pipeline
  checkout       Checkout a built artifact
  fetch          Fetch sources in a pipeline
  help           Print usage information
  init           Initialize a new BuildStream project
  pull           Pull a built artifact
  push           Push a built artifact
  shell          Shell into an element's sandbox environment
  show           Show elements in the pipeline
  source-bundle  Produce a build bundle to be manually executed
  track          Track new source references
  workspace      Manipulate developer workspaces
   
$ ls /mnt/buildstream-workspace/projects/buildstream/
buildstream	  BuildStream.egg-info	contrib  doc	      MAINTAINERS  MANIFEST.in	__pycache__  setup.cfg	tests
BuildStream.doap  conftest.py		COPYING  HACKING.rst  man	   NEWS		README.rst   setup.py	versioneer.py

Acceptance Criteria

[//]: # After installing buildstream, bst command should be running even if the source directory it has been installed from changes or disappear.