fix(npm): correct package_store_prefix_len for links in external-repo sub-packages

[oscar-meroton]

When calculating the package store prefix truncation with external modules (i.e. a repo name is included in the path), an extra slash after the repo name was unaccounted for, resulting in incorrect symlinks (and subsequently ERR_MODULE_NOT_FOUND error during builds).

Below is an example of the logic used to calculate the truncation length:

# ../[com_github_example_repo+]/[example_package]/[node_modules/.aspect_rules_js/]foo@1.2.3/node_modules/bar
#    |        repo name       | |  package name | |         store prefix         |

path = "../com_github_example_repo+/example_package/node_modules/.aspect_rules_js/foo@1.2.3/node_modules/bar"
truncate_length = len(repo_name) + len(package_name) + len(store_prefix) + 3 # = 72
truncated_path = path[truncate_length:] # = "s/foo@1.2.3/node_modules/bar"

The extra s in the beginning of truncated_path should not be there and is solved by changing the + 3 to a + 4.

[jbedard]

@oscar-meroton sorry I didn't see this till now. How can we reproduce this in a test?

[oscar-meroton]

@jbedard Hey, sorry for the delay. I'll try to look into reproducing this in a test later this week.

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