7.8.x Fix bug with task name as substring of task name.

[hjoliver]

These changes close #3664 - 7.8.x sibling of #4426

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.py and conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required.

[dwsutherland]

Could reproduce the bug on my system:

(cylc7) sutherlander@cortex-vbox:c7simple$ cat suite.rc 

[scheduling]
    initial cycle point = 20210101
    [[dependencies]]
        [[[R1]]]
            graph = "foo | xfoo => bar"
(cylc7) sutherlander@cortex-vbox:c7simple$ cylc7 val foo
Valid for cylc-7.8.8-37-gf7a92
(cylc7) sutherlander@cortex-vbox:c7simple$ cylc7 val suite.rc 
Valid for cylc-7.8.8-40-g14bff

But works with changes.