When in umbrella mode (or through multiple profiles), users can specify
macros for EDocs based on either the {def, ...} or the {macros, ...}
arguments.

This patch replaces the prior options merging for umbrellas to use the
rebar3 tup_umerge utils to remove identical duplicates while preserving
correct ordering, and manually merges the {macros, ...} definitions
while ke eping the correct precedence rules since these appear (given
their behaviour) to be all individually extracted and passed as `{d,
...}` to  the compiler so that epp expands them. This compiler
function freaks out on any re-defined macros and explodes.

Do note that the macros with `{def, ...}` are edoc macros and do not
suffer from that issue, safely deduplicating multiple definitions.

Fix crash when a dependency is missing app.src file

Patch up and add tests on #2112

This adds an additional loading and merging of options for EDoc using
the values from the top-level along with those specified in the
rebar.config of an umbrella application.

The app-specific config values are prepended to the global ones; this
can likely cause some problems with manual path handling, but is
unlikely to happen in practice and the rest seems to work fine based on
order

Fixes the issue in #2114

This allows to reduce the number of noise to only checking deps' app
files when they're built, rather than on every run.

Since main apps and checkouts are still compiled every time, the linting
takes place there and then with a higher frequency.

Overrides should apply to a layer below where they are declared. This
patch makes it so if the project root is an application (i.e. it isn't
'root' and therefore not an umbrella project), we omit applying
overrides in rebar_app_discover.

This in turn required changing a bunch of tests, because all the tests
worked with the idea that all overrides applied to all apps to validate
that they get inherited properly. The changes re-structure the cases so
they are written with an umbrella app, demonstrating that the changes
stick.